The latter bolded text is the reason for the former. They don't care about a principle like freedom of association. When they claim that they do it's only to appeal to those who hold that belief but who are also too dumb/naïve to understand that they're only being presented with the idea because the presenters are cynically manipulating them with it.
They're pissed that people can still publicly see the stupid things they post. The "bad" actors that they talk about are usually not hyper malicious internet Nazis, they're just people who will archive their dumb shit and (correctly) call them retarded as you said. It's way easier to gaslight your enemies and the public at large when they can't provide proof of your stupidity. They want a monopoly on who can be the butt of the joke, who can have their history combed through to point out hypocrisies and wrongdoings, etc.