And once that happens, that legitimizes DHH firing Basecamp employees who vote for Trump. I understand that Thiel's involvement with YC is not "employment," but the point is that 1) yes, people are suggesting excluding Trump supporters from employment and 2) high visibility people doing so would have a massive ripple effect on everybody else. It's just another thing that goes from "that's insane and you can't do that" to "these other companies have done it, so it's no big deal."
You can construct a slippery slope out of any argument. I can get you to "DHH fires Basecamp employees" from "refusal to donate to Donald Trump" in just a few more steps. So that argument is not compelling.
It's more productive for us to concentrate on the arguments that are actually being made. You don't have to, of course, but I don't think you'll get very far otherwise.
