Apple’s 15% to 30% for creating the platform and only charging for paid apps is nothing by comparison. And Apple does not even require it as Spotify could launch to a login screen and give ZERO to Apple as a free app.
I am pretty sure the policies say that if you "should" be using Apple payment processing, and you choose not to, you're not even allowed to provide any mechanism in-app to get the user to your own payment system. Therefore, you can't have a Web page open in your app to do payments, you can't have a "click to launch Safari and pay" link, and you can't even have a URL to copy/paste into Safari.
Basically Apple is using UX against the competition. Of course Spotify can (and does) charge people outside their iOS app and gets to keep 100%. Apple is just making the UX for that scenario difficult.
Here's an interesting thing to consider. Let's look at the possible options Apple has/had to deal with subscription apps:
1) "If your app provides paid services to iOS users, you have no choice - you MUST USE Apple's payment system. You're not allowed to let people sign up outside of the app. Or at least if they do, then they cannot be allowed to use the iOS version of the app." (Didn't Sony do something like this?) This to me would actually give a much stronger argument to an antitrust case.
2) "If your app provides services to iOS users,
and the users want to pay within the app, they have to use the Apple payment system." This is Apple's current approach.
3) "If your app wants to use the Apple payment platform (IAP/IAS), you have to pay 30%, but if you're willing to set up your own platform you can use it in the app. You just have to build your own UX and your own payment engine. We won't let you use the Apple payment UI platform." This is what I think most people want.
Options 1 and 3 are sort of the reasonable extremes for Apple's policy. Notice how #2 is right in the middle. By not taking either extremist position, Apple probably sees it as a compromise - we're not
forcing you to use our platform, but we're going to make it bad UX if you choose not to. Also, It also feels like it's
just enough to satisfy the idea of "not anticompetitive", because the issue here is one of UX, not one of force. So what it ultimately comes down to is Apple knows people like good seamless UX, and is requiring you to use their platform and pay their tax to benefit from their UX.