It's actually written into GitHub's Open Code of conduct. (The 'Open' Code has not accepted any changes for quite some time even though this particular issue was under discussion [1].) The Open Code expressly excuses discrimination against some races and genders, specifically by explicitly ignoring any critique which might fall under a 'reverse'-ism; 'reverse-racism' or 'reverse-sexism' [2]. This line in the Open Code was previously discussed here [3].
[1] https://github.com/todogroup/opencodeofconduct/issues/82
[2] https://github.com/todogroup/opencodeofconduct/blob/gh-pages...
[3] https://news.ycombinator.com/item?id=10043668