But how is how Peter decides to spend his personal wealth and his political opinions on-topic?

Because this isn't just a private matter. Peter is very vocal about his vision of society - in which tech plays a big place - and this donation definitely gives some interesting color to that vision.

I would say so if he's a YC advisor. He doesn't have to justify his actions but it would be interesting to understand or at least try to understand his point of view. My guess is it has to do with taxes.

What people choose to do with their person wealth is important, when they use their wealth to influence elections.

Peter is free to support whoever he wants. Just because he is associated with YC, does not make his views YC's views.

And Sam Altman's BFF

Edit: In case you don't think that's relevant, go read this: http://www.newyorker.com/magazine/2016/10/10/sam-altmans-man...

From the article: "If the pandemic does come, Altman’s backup plan is to fly with his friend Peter Thiel, the billionaire venture capitalist, to Thiel’s house in New Zealand."

Do you really think that Thiel's advice has no effect on Mr. Altman's decisions?

Wasn't aware of that. Sam's political views appear to be directly opposite of Peter's then if his Twitter is any indication.

That's why it's surprising. They're "I'm gonna spend the rest of my life with you if a world catastrophe happens" kind of buddies, according to this NewYorker Profile:

http://www.newyorker.com/magazine/2016/10/10/sam-altmans-man...

Their ability to remain friends despite political differences speaks very well of both.