Censorship of your freedom of speech really only applies when the government is doing it. Github is not the government and, by contract law, you waived your right to freedom of speech when you agreed to their terms of service. If that truly does concern you then you're free to move your code to a new repo, or host it yourself.

Unless Github decides that "anything goes" on their servers then it's going to be a judgement call sometimes. Using the word 'retard' in this context might be seen as harassment, unlike using profanity. I personally think Github made the right call here.