Pretty much all open source projects on GitHub rely on section D.6 of GitHub TOS (when you submit PR, it's licensed under project license and you have a right to do so).

TOS are not just fluff and paper, but legally binding. Granted, when there is a conflict between two conflicting requirements, it's never clear cut, but it's not just "GitHub will be mad."

Any project I've been involved in has its own explicit license, that allows such things (really allows much more, I don't touch anything closed source).