And most importantly, it's not 30% for Apple Music.
What do you mean it isn't? Apple Store could trivially take a 30% cut of the Apple Music revenue, and the only thing that would change would be the distribution of revenue between Apple Music and Apple Store in Apple's revenue reports. The total revenue would be the same.
Exactly, but Apple can run their streaming service as a loss leader because they still get to book the 30% revenue on the store side. Spotify can't recoup that 30%.
This effectively gives Apple an extra 30% margin for their streaming music service.
What do you mean it isn't? Apple Store could trivially take a 30% cut of the Apple Music revenue, and the only thing that would change would be the distribution of revenue between Apple Music and Apple Store in Apple's revenue reports. The total revenue would be the same.