Apple are not abusing anything. They are shouldering all of the burden of maintaining an operating system that works well with millions of Internet-connected devices, maintaining an online store (with all of the customer service and payment processing infrastructure that requires), maintaining the means of distributing software to users (who can redownload the app for free as often as they need to).
Apple provide all of this whether or not you charge your users (and thus whether or not they get a cut of your revenues) - they make zilch from Spotify's millions of ad-supported users. So, the app store fee is no more a "tax" than rent is a "tax" on retail stores - it's a basic cost of doing business.
Spotify are being very misleading in their statements about all of this. They want you to think of them as the poor little David facing down Apple's Goliath. It's a crock. The real Davids are the hundreds of thousands of musicians and songwriters they pay abysmal royalties to (if they pay them at all, here's a fairly recent, high profile example of them not doing that:
https://www.cnn.com/2019/08/22/entertainment/eminem-spotify-lawsuit-trnd/index.html ). Spotify are not alone in poor royalty rates, but Apple, Tidal and Napster are all ahead of them:
https://www.digitalmusicnews.com/2018/12/25/streaming-music-services-pay-2019/
They are still one of the most popular streaming services, a position they would not enjoy without Apple. iPhones and the app store (and Android phones and Google Play) created an audience and a business model for them that desktop streaming alone would not have done (last I heard, ~60% of Spotify's traffic came from phones and tablets).
I'm not saying 15% or 30% are _fair_ because I have zero detailed data on the matter. That said, 0% is also not fair, which is what Spotify seem to want. Added to how they treat artists, it seems they want to have their cake, eat it, then take a giant dump on everyone else.