Maybe Shane was drunk and grooming kids? We know the former was definitely a problem, and it got him shitcanned.
But seriously, now that you mention it, it is weird that they would know this person is a problem, and yet keep letting him in. Cons are private events. Attendees have no elaborate due process rights when it comes to them being there. If the staff wants you gone, you're gone. You pull their badge, and call the cops to have them trespassed off the property if they refuse to leave.
The only time I have ever heard of a convention hiring outside security was when I learned that Otakon hires (or, at least at the time) armed guards from an armored car company (i.e. Brinks) to pick up the significant amount of cash they collect. That's reasonable, IMO. Businesses that deal in a lot of cash do that all the time.
I mean, I'd like to say A-Kon is dumb if they did any of this, but I find it more reasonable to just presume Shane is full of shit. He's once again claiming something in an attempt to make him look saintly, but his former convention look bad. Glug glug, Shane. Eat a shoe.