Probably a contract clause somewhere that forces them to make that payment method equally available to other payment methods.

Would be good to have continuous reminders about what kind of assholes these payment processors are.

Merchants need payment processors to make money, and losing one makes a big dent in the sales. I won't get a Mastercard because Visa isn't accepted anymore.

If they say that X processor is a dick and lose them, they are worst for it.

Not sure how viable is that, card payment is very monopolised system, and those few get a say on what to do.

Though i guess steam can bypass it by having people reload their steam wallet then buy using that.