Spotify has been at the forefront of the battle against Apple's App Store practices, filing an antitrust complaint in the EU back in March. Apple charges a 30% fee on almost any transaction that occurs to purchase digital goods.
Spotted by Der Spiegel, Apple says that Spotify is blowing things way out of proportion when it says that Apple's 30% cut could cause it to raise prices. The reason is that Spotify only pays App Store fees on 680,000 of its over 100 million subscribers, and it's only 15%, the cut that Apple takes for recurring subscriptions.
Spotify only offered the option to sign up for its Premium membership through Apple's services from 2014 until 2016. In other words, there are no new members for Apple to take a 30% cut. Basically, Apple's 30% rate caused Spotify to give up on App Store sales, as many have done.
Other companies like Amazon have their users purchase all of their digital goods through the browser. Once you've done that, you can consume them through their iOS apps. For example, if you want a new Kindle book on your iPad, you'd have to open Safari, buy the book from Amazon.com, and then go back to the Kindle app to read it.
It seems pretty clear that the larger issue here is that it's not financially feasible for companies to offer services through Apple's in-app purchase system. It's a disservice to users as well, forcing them to jump through hoops just to get the content that they want.