Apple account is not the same as login with Apple. IAPs have always been tied to the Apple account. It’s a pain in the ass to handle all the cases. It’s not any easier on Android either.
Right, but Sign In with Apple is tied to your Apple account. I have no way to make an IAP autorenewable subscription and tie it to “bob” in my system. It’s tied to the iOS Apple account.
You can’t tie it to bob. You have to look at the receipt and reward from that. So bob logs in you look at the receipt and bobs your uncle you have the subscriptions.
And then what if, on your service, `bob` signs out and `alice` signs in? Then `alice` not only has `bob`'s subscriptions, but she also can't buy her own.
Apple doesn’t see it that way. Apple ID has the subscription, not the the user account.
Apple has supported switching between multiple user accounts on a device for years, but since you're not a school or a company, you can't have it. Buy four iPads.
It's pretty stupid.
Technically, you still have that capability. After the user is authenticated with their Apple ID, you can present a profile to provide username/password/email.
Yep, but then all username/password/emails that log on using that device share the same subscription IAPs—namely, those that belong to the Apple ID that's sign in to iOS.