But why do they care? You're paying the transaction fees. They're getting their money. If anything isn't it good for the payment processor to get more transactions?
It's a real transaction. Nobody was deceived, the money really changed hands, and the payment processor got their fee.
If I own a physical shop I'm allowed to buy stuff from it if I want, why isn't that also the case for an online store?
I'm not saying it isn't against the ToS, but I agree with OP it isn't obvious why it should be and it seems like almost everybody would test at least one real transaction at some point.