No one is forcing people to use Apple devices. That's not what this is about.

It's about other services trying to reach potential customers that happen to be using an iPhone. Spotify has to go through the App Store if they want to reach any customers on the second largest mobile platform. And Apple themselves have a lot of advantages concerning integrating their own music streaming service into the OS while Spotify is limited by the rules Apple sets, including taking 30% of any subscription made through the App Store.