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.
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?
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: