I currently subscribe to Spotify, use the iOS Spotify App, and do not make payments through in-app payments nor do I pay the 30% Apple tax. So it is circumventable, and Spotify is just "optimizing" their revenues here. So at least compared to Steam, Apple's platform is actually more open since you aren't required to pay through Apple's platform. Though I'm not sure on the current requirements for in-app purchases, so correct me if I'm wrong.