Github has the right to remove any account it wants because terms of service generally allow that kind of stuff without repercussions (though they need to explain why in certain legislations by law).

However, if Github is actively curating content, that means they're also responsible for the stuff they do leave up. The argument that a company isn't legally responsible for its users content relies heavily on the company having little control over said content.

This direct influence is what got the thepiratebay guys and the megaupload guys arrested, or at least what made the case persuable enough to put effort into it.

Id it's true that Github is actively removing account that post things they deem as racist, then I will judge Github on any older content on the platform not taken down. There is a lot of weird shit on Github that's apparently okay if a stupid mod for a video game is considered as a reason to refuse service to someone.

Companies are free to choose what they do and do not host, but that freedom is a double edged sword.