Just because Apple can do as they please, doesn't mean they ought to be allowed to. Antitrust is a useful tool for when players end up controlling monopolies and using them in anti-competitive ways.
It’s important to define exactly what is “anti-competitive.” If Apple banned Spotify or Deezer, that could be considered anti-competitive. But they aren’t. Netflix has original programming, is that anti-competitive? Don’t Netflix originals compete against non-Netflix shows for streaming revenue? Are there alternatives to Netflix? Yes. Are there alternatives to the App Store? Yes: Spotify could, for example be web-onl…
On the other hand, you say Apple is free and clear for undercutting Spotify with their own product in a way that Spotify is incapable of competing with.