There is. By only allowing purchases by in app purchases, Apple can force the app developers to be fair in their return/refund processes. There are tons of scummy companies and developers want to take money from the customer and not return it if the customer is unhappy or even when they are exercising their legal rights and hope the customers to not bother for their 1 to 5$ purchase.

Believe me, the rules do make sense.

And the many stores we can trust are more important than hypothetical abusers. Hypothetical abusers can be addressed in other ways, for instance Microsoft shuts down malicious servers and entities using lawyers and law enforcement.

Law enforcement will do a lot of good if the author isn’t in the US....