In fact, if GitHub were an American shopping mall, they would not be allowed to stop people from wearing T-shirts that mocked feminism, because of they way the first amendment is interpreted.
I don't think GitHub should remove material just because it differs from their political viewpoint, or from the viewpoint of people who are able to cause the most trouble for them. Of course people are always free to criticize the repo and its authors if they don't like it.
Is that Github's business to host political/comedic content? Not exactly - they do code hosting. On the other hand, code is just text files and they can host any form of data.
Is it their business to police content? That depends on jurisdiction - for example, in the USA, much depends on obscenity or hate crime laws. In Germany, if something were deemed to be Nazi propaganda, it would be illegal.
Obviously, none of this applied here - instead, the internet 'progressive' outrage machine created a PR nuisance for GitHub that needed to be addressed somehow. It creates an unfortunate precedent, and points to the trouble that a person or group can have when they don't control their own content and servers.
Sites are free to set there only rules, but sites like github can also be considered as a public space, and so I think these sites have a moral obligation to make these rules objective and not politically biased. What is troubling is that, whether the rules explicitly say so or not, they tend to censor non-pc material. I'm sure progressives would have no problem seeing the issue if the censorship was the other way around.
