If you are actively being stabbed, the legality of it is pretty irrelevant to your situation. It is about a place that hosts a large part of the open source community. Should a private company have power over all their contents?
The term isn't even usually applied in contexts like this one. This here is usually called moderation or content policy or editorial standards etc. And most people don't consider it a bad thing.
But certainly are arguments for largest market share holders to have some kind of neutrality standards, like implemented in common carrier or net neutrality regulations. And in the other direction: at least in the EU there has been a big push to hold forums responsible for things like defamation and hate speech. eg http://arstechnica.com/tech-policy/2015/06/shock-european-co...
This is false. Facebook's ban on photos showing female nipples is constantly referred to as censorship.
http://www.huffingtonpost.com/lina-esco/facebook-war-on-nipp...
But it's still qualitatively different from government censorship, which is imposed on third party media.
This is a weirdly common kind of thing for people to say. I don't understand what the point is supposed to be. It's still a bad thing. We have prohibitions on the government doing this because it's bad. We don't have prohibitions on people censoring third parties in their own homes, because that would be worse. This is still bad for all the same reasons it would be bad if the government were doing it. We're not criticizing github for doing something illegal. We're criticizing it for doing something wrong.