Like what?
It isn't fixed at $7, it's fixed at 70%. It's $7 paid per subscriber when revenue is $10, but if 30% goes to Apple they pay 70% of $7.
It actually is fixed for paid subscriptions. They have separate deals for their $5 student deals, trial period and the $0 ad-supported subscription.
Let's say Spotify would charge $9.99 even through the App Store. That would mean $3.00 would go straight to Apple. 70% of the $9.99 (that's $6.99) still goes straight to the publishers. That leaves
exactly $0.01 for Spotify to run their business... which you can imagine is not enough.
At the same time, Apple has its
own music streaming service, also at $9.99. Roughly $3 goes straight to them to run their service, instead of the measly $0.01 Spotify gets. Let this sink in, Apple is not merely offering a platform to Spotify for which they ask a 30% cut.
Apple is actively competing with Spotify - and when that's the case, Spotify should have equal opportunities without limitations.
Yup, Spotify - a company that isn't making any profit - must pay a 30% cut for every subscription it sells on the App Store to Apple, while Apple - a company that makes tens of billions of profits each year - has none of these extra costs.