There's a substantial difference between forcing someone out because of the potential that other people would shun them for political reasons vs because they are taking a stand that you think would do irreparable harm to your industry. The first is pure politics, with only the indirect consequences on your company that all businesses could face. The other is a piece of pointed legislation whose primary goal will disrupt your industry.
Silicon Valley already has tremendous diversity issues and a reputation as out-of-touch. If the future of America and the world only gets more diverse, which seems to be the case, then YC has a vested interest in being on the right side of history.