Earlier quoted context omitted.
Seems pretty fair, since if it was easy for an artist to undercut Spotify, they'd be doing it.
Just like the application developers on Google Play, Apple's app store, and Steam, eh?
Even Google Play is iffy, as developers could distribute through 3rd party app stores or sideloading, though Google makes that harder than it should be.
Apple is really the only one of the 3 you mentioned that forces a 15/30% cut if developers want to distribute an app on their platform.