There is clear precedent for YC taking a political stand by uninviting investors. Only five years ago, Paul Graham was willing to uninvite investors from Demo Day based on their support for the Stop Online Piracy Act (SOPA). https://techcrunch.com/2011/12/22/paul-graham-sopa-supportin... Paul asked, "If these companies are so clueless about technology that they think SOPA is a good idea, how could they be good invest…
He is attempting to undermine democracy in the country and I'm not sure you need a single additional reason to object to anyone who supports him.
Thiel, of course, does not think "freedom and democracy are compatible": https://www.cato-unbound.org/2009/04/13/peter-thiel/educatio...