Let's imagine Amazon can sell eBooks via their Kindle app.
Now, let's say I'm a writer, and I've written a book. I want to distribute it via the Kindle app but I don't want to use Amazon's payment method; I want to use PayPal, for example.
Can I, as a writer supplying a digital product to Amazon, insist that Amazon use PayPal for sales of my book, or do I have to use whatever system Amazon has in place?
Is Amazon's cut of an eBook sale similar to Apple's App Store cut? Are Amazon charging customers only the credit card charges, or are they charging for maintaining the eBook on their systems, marketing it, and development of the systems allowing that eBook to be updated, maintained, listed etc.? Are Amazon allowed to profit from their Kindle app?
As I see it, that's what Spotify (and some users on here) are asking for. You want developers to be able to insist Apple use a specific payment system for sales of their app.
Will they then also demand that the app or digital content, download or whatever is stored on specific servers? Who pays for that?
If Apple is forced to allow third-party payment systems, should they then also allow apps to be stored on third-party servers? What about paying for the bandwidth used; should that be determined by the developer?
If third-party payment systems are used, do Apple then reduce their cut to 28% for those developers? I mean, if they aren't supplying the payment system but are still supplying all the rest, can they still charge for that, or is the point here that Spotify want Apple to get zero money for all the work they put into the App Store and to cover its associated costs? If Spotify want the entire 30%, can Apple then charge Spotify for providing the services to the downloaders of their app, or are Spotify taking over everything, payment, distribution, marketing? Can Apple still charge for developing the App Store systems and improvements?
Breaking this down into "payment systems" leaves way too many questions about the rest of it. Apple is providing a service that encompasses all the above, and charging 30%/15% for it. Is that so bad?