To put this in 19th century anti-trust terms, the manufacturing company owns the railways and charges the competition a toll to transport their goods. It's a clear cut case for Spotify, from a legal perspective.
To repeat my question asked of someone else in this thread: Apple charges e.g. Mophie a fee to feature their products on the Apple Store online. Should Apple be required to pay the same fee for featuring their own products there?
What Apple is doing is a direct violation of the Sherman Act of 1890, specifically Section 1, in regards to artificially raising prices by restricting trade or supply:
> Every contract, combination in the form of trust or otherwise, or conspiracy, in restraint of trade or commerce among the several States, or with foreign nations, is declared to be illegal.
Apple is big enough to pull their products from Amazon. Taking it back to the 19th century, Apple owns their own "railway". The real quandary comes in if Amazon gets so big that there's no way for a small supplier to sell a product in mass without going through Amazon and paying their premium. I don't think we're anywhere near there yet.
Amazon also built their own Amazon store, to get away from the fees of Google's equally Sherman Act violating store, and in the process potentially violated the act themselves (Prime Music).
Edit: I just thought of another, slam dunk case, this time for Amazon, because Apple restricts purchasing Audible books in the app, even when using existing book credits.
I am genuinely curious, does Google Play violate the Sherman Act in the same way that the Apple App store does or is it a different clause?
It seems like allowing users to fairly easily side-load apps (ex. Fortnite) and the presence of competing app stores (Amazon App Store, F-Droid, etc.) should alleviate some concerns.
Exxon doesn't force Shell sell their gas in Exxon stations.
And if you don't, you're out of luck due to the increasingly hostile restrictions on what you're allowed to even mention in the app - at least according to the website.
Can't say I'm 100% on Spotify's side, but to me it doesn't seem like Apple has anything stand on here, ethics-wise. Microsoft and the IE case comes to mind.
But Spotify's argument seems to be "you have to pay the same price as everyone else ... except Apple itself."
And why should it change just because Apple introduced a competitor? Apple charges Spotify the same before and after they launched Apple Music.