So, for $99/year, Spotify is given a platform to host their app, and provide millions of downloads of their app into a very lucrative marketplace with qualified buyers. That's essentially free to Spotify. It certainly costs Apple quite a bit more to build, maintain and host the marketplace than Spotify is paying to Apple.
Spotify, in large part likely due to this free platform that Apple provides, has managed to gain 30.5% of the streaming music business, nearly tripling Apple's marketshare of 13.7%.
EU citizens, who seem incapable of managing the overly burdensome task of managing their Spotify subscription in a web browser (also available on the iPhone), are crying that this arrangement is somehow unfair to Spotify. Which boggles the mind—Apple is giving a lucrative platform to Spotify for free. And crying about it.