Actually, yes, Spotify does have to be available on iOS devices if Spotify chooses to. Apple will be slapped with aggressive enforcement action if they pull something like that.
You're essentially saying if a crisp manufacturer wanted to sell in Asda, but Asda didn't want to sell them, Asda could be slapped with enforcement action for not selling the crisps?
That would be a huge breach of anti-monopoly laws - you are abusing dominant position in the market to fuck over your competition (on the music streaming market).
Apple isn't a deity.