Apps that allow the purchase of real-world goods like eBay and Kickstarter seem to get a free pass, which makes it all the more bemusing why media platforms like Spotify have to pay the 30% levy.
Its not bemusing: if Apple didn't charge a levy on in-app subscriptions, you can bet that, suddenly, everything in the app store would be free or 99 cents - but would turn out need a subscription to be of any use. That would make the App store non-viable for Apple. Allowing the purchase of 'tangible' goods doesn't risk that.
Remember when they got Microsoft for putting their browser bundled with Windows?
You have to remember that, back in the 1990s when this issue started:
(a) Microsoft had reached a near-monopoly position in the PC market - I forget what the market share figures were, but it was in the 90% range. They only got that by pulling the whole scorpion & fox thing with IBM (except in their version the scorpion learnt to swim) and hopefully we will not see its like again.
(b) It wasn't, then, common practice for a PC operating system to come with fully-featured applications software. Prior to Internet Explorer, Netscape, Opera et. al. had a business model based on supplying web browsers for Windows, which was wiped out overnight (so they claim*) by Internet Explorer. The case took so long that, by the time it was over, a web browser and email client was an expected part of any desktop operating system.
Bottom line is that Apple may be big in mobile, but they don't even have a
majority in the phone market, let alone a monopoly. If they effectively kick Spotify & Google off of iOS, there are still a lot of Android phones, Chromecasts, Amazon Fire dongles, smart TVs etc. out there that people could switch to.
(*The fact that there was always plenty of good, free internet software around, including free versions of Netscape, probably spelled doom anyway, but Internet Explorer certainly delivered the
coup-de-grace).