Side note: most people who apply to YC are probably not very public with their politics. Political minorities could do the same and not worry about repercussions for their beliefs.
There's only so much they can judge by in the small amount of interviewing they do. It's much like hiring
Secondly, stop thinking of free speech as a right that protects us from the Government. The Government is rarely the issue in the US. The free speech ethos should be something in my head that protects you from illiberal outbursts or actions on my part.