I think it's because Stripe has for a long time branded themselves as being priced simply. Reading that there is a price increase, I think, warrants a noticeable reaction, since it could mean a new 0.5% cost for every business relying on them.

That can be a lot of money for some people, and in general, I'm a bit disappointed that I found out about the Billing product increase through HN. I would have expected to get an email from Stripe, maybe a month ago, saying "Hey: our rates are going up on 12 November"