Au contraire. Spotify, like Netflix, could offer sales of their service outside of the the App Store and not pay the 30%. There are many businesses that have apps on the Apple App Store that do not offer financial transactions through the store.
That's beside the point. Sure, not offering transactions through the store is a possibility, but it still puts those developers at a disadvantage when at the same time Apple can offer transactions through the store without paying a fee.
The entire fact that Apple dictates a singular way to access apps is just wrong.
Imagine a single company controlled the only way you could install software on your computer. That would sound ludicrous, wouldn't it?