But they don't offer a porn app. That's the big difference here. The fact Apple offers their own competing service is the crux of the issue. They are using their platform as leverage to get more of the customer's money without necessarily competing on the merits of the service. See also Apple Maps. The default map program is Apple Maps, even though it is nowhere near as good as Google Maps, and you can't change it. Apple isn't competing on the strength of its program/service, it is competing on platform lock-in, and that's wrong.
They had the 30% cut way before they had Apple Music. The point of the 30% cut is simply that if anyone, (not just spotify) is selling anything in their store they want a cut because they created eco-system, they bear the trust for the payment scheme if anything goes wrong, passwords, encryption, branding etc.. Its their devices that they spent billions making that brings the lucrative, high income customers that Spotify wants. Apple want compensation for building that eco system with their own money and risk.
Where you can add to apple's bottom line in a better way they cut a deal (Microsoft Office etc... pay less I believe, Amazon because of the Apple TV). Where you cant (spotify), hell no. If Amazon or MS pulled their apps thats a big problem. If spotify pull their app, so what, they will sell you Apple Music.
As for the maps, when google maps was the default map did anyone complain that Nokia maps wasnt default? No.
Then google asked for more data on people and Apple said no and had to build their own maps.
It wasnt something apple made to make money. Apple makes all its apps to make sure its own customers have a default level of service and expectation out the gate. Because you pay alot of money for apple and thats what you should have.
And they dont want customers getting used to default options that they cant even guarantee will be on their devices in a few years time (i.e google maps) or back in the day when Adobe wouldnt make some of their apps for mac or were destroying the performance with Flash.
There is a history and reasoning behind all of this.