This was being asked as long as I remember (~15 years now?) but the conversion commissions were around 2%-5% at most. 15% is egregious.

At least in Brazil, it was very rare. In the last 3-4 years, it's almost every time you pay. And you have to grab and hold the payment terminal (especially if you're using tap / contactless payment) so the cashier or waiter, trying to be helpful, doesn't click the wrong button and cost you 15%.

2% is already excessive and 5% is quite egregious.

Plus, the point is that you're asked whether you'd like to pay more for something, where there is no benefit in it for you nor a public benefit etc.