Bingo. Apple gives Spotify access to a marketplace, app distribution, software development tools, access to all iOS users etc. and at this point Spotify has paid $0. Even while they make money off the free ad supported version of Spotify.
What they want is access to Apple’s in-app purchase system at a rate similar to merchant fees (1-3%). The issue with that is Apple’s system offers way more than just processing a transaction. It processes the transaction, deals with customer billing support, handles payment issues, stores credit card details, and on and on. It is not just a payment transaction, it is a platform.