Unfortunately, Apple has to maintain this rule because otherwise developers will implement workarounds to avoid charging in the App Store altogether.
Remember when eBay charged a commission on the sale price but not shipping? Suddenly $50 items became $1 with $49 shipping -- so eBay had to charge the commission on the shipping too even though it didn't seem "fair".
Same here -- every app would turn free, and charge to unlock features within using their own payment mechanism. Apple's App Store revenue would drop to zero. No developer in their right mind would let Apple take a cut of anything.
As a general rule, app stores simply don't allow third-party payment in apps that don't go through the store, for this exact reason. It's not just Apple with this policy.
Remember when eBay charged a commission on the sale price but not shipping? Suddenly $50 items became $1 with $49 shipping -- so eBay had to charge the commission on the shipping too even though it didn't seem "fair".
Same here -- every app would turn free, and charge to unlock features within using their own payment mechanism. Apple's App Store revenue would drop to zero. No developer in their right mind would let Apple take a cut of anything.
As a general rule, app stores simply don't allow third-party payment in apps that don't go through the store, for this exact reason. It's not just Apple with this policy.