To them, people they trust have made accusations, so they of course believe them. Those accusations keep expanding, and now include 20, 25, 30 women - and there were rumours beforehand, and so many women can't be lying look at Bill Cosby and Harvey Weinstein, and their friends/partners wouldn't lie to them, so of course this is all true.
And to deny that, or even question that, is in current day equivalent to going up to a rape victim and slapping them in the face. Because now, after an assault, a victim's response is just how they 'process trauma', from understandable reactions like not wanting to tell anyone due to shame and fear or wanting to immediately shower even though that will destroy evidence, but also including things like continually acting friendly or pursuing a further relationship with the alleged assaulter or stalking and harassing them for a relationship. So not only are you now expected to completely believe any claims of sexual assault, and you're not allowed to ask any questions about the story of how they were assaulted, but even their behaviour afterwards is considered off the table.
So at the point where it's a woman they trust telling them these things, I get it. They may be incredibly naive about the chance they're being lied to, but your fiancee tells you something bad happened, you'd tend to trust them. But where they start going off the rails is where they then immediately treat everyone who doesn't have a good reason to trust the story as Bad People who can be safely ignored, dismissed and mocked. And how they can take evidence that maybe things just didn't happen the way they think it did and it doesn't matter, because they're busy Believing All Women, doing the right thing.
So they never examine whether the behaviour they're ascribing to their opposition could also come from their side. The Bad People will lie, conspire, defame, act out of malice and greed and bad faith, but none of that is coming from their friends and loved ones. And again, while that might be understandable, they also extend that belief to everyone who agrees with them, which in this case includes a large amount of untraceable, unsourced, anonymous internet accounts, which they should have absolutely no reason to accept at face value, except that it says what they want to hear so they do.
And for Bad People, anything is possible. We see with trooncows all the time how if they're 'feeling unsafe' it's the same as someone abusing them, verbally and physically. And the same thing is happening here, where the hugs and kisses on the cheek on consenting women is being interpreted by other women and creepy and unwanted, which is practically the same as assaulting women. Which means he's assaulted women, and if he's done that then he's done it multiple times - look how many fans he has photos with! Some of them must not want the attention! - and if he's hugged a girl, it doesn't matter if she specifically asked for a hug because girls can't consent, which means he's basically assaulted the girl, which means he's a pedophile...
And that's how it works, amplified by social media. So rumours become facts become 'everybody knows', and it's now a truth you can't deny, like that xkcd comic a few pages back about how a random Wikipedia update becomes considered true just because someone with any perceived authority cites it without checking. It's the Ferris Beuller rumour mill, except more serious and none of them have any perspective because questioning any point of the narrative is akin to denying sexual assault has ever occurred. It's also how you get things like them inadvertently ignoring assaults on children, which would be criminal - because examining the narrative is forbidden. You just have to accept it as truth without question, while also not questioning why the most people have done for years about this known sexual predator in their midst is bitch about him behind his back and spread nasty rumours. They argue Vic is too popular and powerful to stand up to, but there must be at some level where they both think he's a terrible abuser but also he's just that bitch eating crackers, where everything about him is annoying but that doesn't mean you can actually do anything about it. And challenging why none of these dozens of women haven't come forward is forbidden, so they just accept it and move on.
But now the actual legal system has entered. And that requires a standard of evidence beyond rumours and exaggerations, and stuff that 'everybody knows' has absolutely no standing. So Shane and Ron, who have started from a position of trusting a couple of women they have (at least in Ron's case) reason to trust, now have a whole outlandish narrative they've accepted as true and anything that denies it has to be fake. Add to that a real lack of knowledge about how the law actually works outside of fiction and a desire to white knight, plus in Shane's case a real need to be seen as important and part of the cool group, and they find themselves spreading the rumours as truth, and doubling down on this whole fragile narrative they've had to accept.
And also, when they finally turned on Vic and got rid of him, as they thought, that was meant to be the end of it. That anything they did to get him out of their lives might have actually been illegal would never have crossed their minds, just like with Mark Waid vs. Richard Meyer. After all, they were doing the right thing! They were getting a Bad Person who had done Bad Things away from them, and would presumably not have to deal with them again! That any of the people they had managed to align themselves with might have anything other than pure motives couldn't be possible, because they were Good People. That any of the accusations might not hold water - well, everyone knows it's true. Everyone! So warning cons, pulling strings at companies, that's all necessary to protect women and get rid of a bad apple.
It's why it's so much of a religion. Because they've developed a principle that must never be questioned, just accepted on faith alone. It is something no-one would abuse or misuse - that's only what Bad People do, and we're Good People. And so what starts off as trusting one person metastasises into unquestionably accepting anonymous claims, attacking anything that challenges your faith, and doing everything within your power to bring down your opponents ... except leave the house, apparently, since doing something like making an official police complaint would involve actually, legitimately, thinking about the crimes committed and the accusations, and maybe, just maybe, realising it might not be as true as you think.
They seem to think that real life is like Twitter, but Twitter isn't real life. And it really feels like that's part of what makes this so exceptional. Because anything that happens online doesn't really count, or reflect your character, and you can say what you want and the repercussions are only meant to hurt the Bad People, not the Good People. They can say what they want and it's fine, but any backlash is horrible, unprecedented harassment that was organised to hurt them. So we see Ron and Shane and a few others constantly calling other people things designed to hurt them and their reputations, to have consequences in their actual life - and that's fine, to them. But if anyone denies it, or threatens legal action? It was only on the internet, it's not that serious, they'd have to be bluffing because who really thinks tweets matter that much?
Basically, the cognitive dissonance starts from one small, reasonable place, and follows a path of zealotry, ignorace and black and white thinking to a point where they have absolutely no perspective on what they're doing or saying:
They are Good so will prevail (even though they'll go on about how Evil is winning constantly and bad things are happening).
They know the accusations are true so they have nothing to worry about (even though Nick and others have been telling them repeatedly that the accusations are mostly moot and they only have internet rumours and likely a couple of women who have been lying to them).
The sexual assaults have been going on for years and so stopping them by getting Vic fired is the best course of action (even though if they really, truly believed Vic was guilty of what they're accusing him of, especially with underage girls, and didn't put a stop to that they are not only morally responsible for abetting a molester, but criminally liable for not doing anything about it).
If only Vic had apologised and let it all happen, lay low for a bit, then things would be OK (even though ruining a career and future income isn't something they are willing to accept for their own actions, that course of action is only beneficial to them but not Vic, and they are being disingenuous at best if they expect that someone like the monster they have made Vic out to be would make it acceptable to have him back at a con without being constantly shamed and sent to Coventry).