If you want to put your product into the distribution channel that Apple spent shitloads of money to create and operate, then you have to pay their fees and that means you can’t engage in a practice like linking to an outside signup page that is solely for the purpose of circumventing the fees you rightly owe for use of the distribution channel.
Nobody has to distribute your app for free. You don’t like the prices Apple charges for it? Go to Android, or make your own distribution channel, whatever.
Spotify is free to not participate if they don’t like the pricing.
Spotify already has their own distribution channel (sign up on their website), and Apple is prohibiting them from mentioning it on iPhones and instead forces them to use Apple's distribution channel, which comes with a hefty price tag.
So if they want to distribute their free app on the iPhone, they are welcome to do so, as long as they don't mention that people have to go to their website to sign up for the premium service.
We can still argue about whether Apple is allowed to behave this way, but at least we'll be arguing about the right thing.
> “Spotify already has their own distribution channel (sign up on their website)”
Yes, and if they don’t like the prices for oveying the rules of Apple’s distribution channel, they are free to remove the app from Apple’s app store, and notify customers that they cannot use the service on iOS devices.
If Spotify deems 30%/15% split is too much to be able to sell their services on iOS, they don't need to participate in that market, they can continue selling their online streaming services on the Web and Android and and spend the money they would've paid Apple in distribution costs in more advertising/marketing to acquire new Customers.
If the App Store revenue share is so lucrative they can also consider that as an opportunity and are free to build and sell on their own platform, invest their resources in building hardware + mobile devices, OS, Apps + software, dev tools, distribution channels, retail stores, marketing/advertising to entice everyone to use it and enjoy the 30%/15% revenue share from everyone else choosing to participate and thrive on it. Or maybe it will end up being cheaper for them and they'll get more revenue because of iOS's larger and quality user base to just pay the App Store fees for being able to offer their commercial services to the iOS ecosystem created with Apple's investments.
Best case scenario is
1) Apple switches their platform monetization to a more direct model where you pay to advertise your app in the App store. This will get tricky because their "Top apps" list still serve as advertising but you can't just remove an app from there because it refuses to pay.
2) Apple removes the restrictions against advertising alternative payment/subscription methods
The problem is that you can't laugh Apple out of the market for payments on iOS devices, because they don't allow other entrants into that market.
Is it because they provide a service in exchange for that 30%, a service that many people are willing to pay 30% for?