Liquor stores are not candy stores, yet they are allowed to sell candy to minors while being prohibited from selling liquor. The principle is straightforward: regulation should follow the product, not the venue.

All pained analogies are both pained and invalid.

iOS is not a liquor store, and allowing people to use other payment processors or even other stores on the platform is not selling liquor to minors.

Note how your analogies immediately fall apart for other platforms like, for example, Apple's own MacOS.