By allowing you to pay via an apple controlled payment method; or (less securely) by contractually obligating them to honor centralized cancellation requests.

I'm almost positive many people would gladly pay extra - certainly temporarily - to be sure no untrustworthy third party gets direct access to their money. The question is merely whether apple gets to force that when both end user and app developer would prefer otherwise, and if there are any conditions on such requirements by Apple.

One way I'd like it to work, us that you can choose to sign up through Apple, and have the service pass their cut % along to you as a greater cost, or sign up without apple's system for presumably cheaper.