What do you censor then though, and who decides it? I think sites like Twitter and Facebook should allow anything unless it's illegal, but that's just my opinion. Harassment is illegal, for example, and so is wishing group X or Y "be killed off" etc. or any threat of violence Planning terrorism, whether from arab or white radicals, should also not be allowed.
The internet has a ridiculous amount of stuff that isn't meant for kids or teens. We don't want to start burning books.
What they censor is up to them. There is no law that says any of these platforms have to post anything or allow unrestricted free speech. They are responsible for what appears on their websites. If they happen to get their asses sued off because of something they allowed, that's only fair..