What stops Apple from updating their terms to make sure that every in-app purchase is reported back to Apple. They can they bill the developer 30% commission for hosting and app vetting on a monthly basis. Failure to pay, suspends the app from the store and on all phones.
There is your "non anti competitive" payment solution, who will also charge you a few percent. The developer got what they wanted but now they have even less revenue.
Biting the hand that feeds IT
