Again, I've watched this happen. The defining thing about the assholes is not that they're right or wrong, but that they dominate the discussion in quantity of posts and use the forum for their own discussions, typically unconnected to the subject of the forum, and can get insulting. Eventually, the people interested in the topic decide that it's not worth burrowing through the crap and the abuse and they move away, leaving the forum to the assholes to use for their own stupid purposes.
This has nothing to do with right or wrong. I've been in situations where I was in a very small minority on a particular topic being discussed. I'm perfectly comfortable in that situation, as long as the discussion remains civil and reasonably on topic. I find those situations good learning experiences, even if I don't change my opinion, and I rather enjoy them.
This is also not a freedom of speech issue. If you are in my house, I'm happy to let you express different opinions, and I'll join in the arguing and maybe we'll all learn something. One of my friends said he had smart friends with weird political views, I told him that coincidentally I had the same issue, and everything was fine. If you get obnoxious or insulting I will take some sort of action, which might well be to ask you to leave, and exercise your free speech rights somewhere else. By doing this, I keep exposed to different points of view (although to be honest it can drive my wife up the wall), and we all stay friends. In other words, I'm not at all coming from an anti-free-speech or everybody-must-agree-with-me angle.
Twitter is a private service, trying to give lots of people a good experience so they can get money out of the situation somehow. If most people have a bad experience, the company dies. For this reason, they have a vital interest in keeping the conversations more or less civil. If you want a different sort of service, feel free to start one. That's what freedom of speech is all about: you don't get to say anything you want on somebody else's site, but you can start your own and try to attract people.