I get why Apple does it. Beyond the money (yes, Apple likes money), it's simple and understandable. It provides a level of confidence for users that payment will be handled properly, not a scam, etc.
But I think Apple is too strict. Here's what I'd like to see them do:
Provide a link to subscribe/buy media. (This is ONLY for digital media, no game stuff, no unlock a full program, etc). When a user clicks on it, they get an alert saying that they are leaving the Apple environment and going to an external store. Proper warnings that the normal protections may not apply. Users can click OK to go through to the website and pay there, or cancel and stay where they are.
One other alternative would be to allow the purchase of subscriptions/digital media with Apple Pay for a lower amount (maybe 5%) Still relatively secure for the user.