> The user has already paid to download my app from the App Store and Apple has gotten 30% of the cut. What users do on my App after that is none of Apple's business, though of course Apple would like to claim otherwise.
If the App Store runs the way you describe, then everybody would offer their apps for free to avoid the 30% cut and also not have any in-app purchases (since those also have a cut). The result would be the user installing the app and having to go to a website (even if it’s embedded in the app in a web view) to create yet another account, finish the signup process, go through a separate (and usually lengthy) payment process to actually buy the app and managing those payments in cases where those are subscriptions.
One can argue on the merits and demerits of Apple’s current system (which needs an overhaul, IMO), but the other option isn’t without demerits as far as users and user experience are concerned.