Yeah, I agree, Spotify's case is pretty compelling.
I'm not a lawyer, but my understanding of the Sherman Antitrust Act (I wrote a paper on this in engineering law undergrad about the Intel/AMD case) is that whenever a company uses its dominant position in one market to attempt to restrict competition in an adjacent market, that's called "tying" and it's illegal.
And this seems like a pretty textbook case of it: Apple has a strong, powerful position in mobile devices, and they're making it harder for Spotify to compete with them for music streaming services.
Apple's argument would probably be "we do this to everyone, our behavior isn't specifically to reduce competition, it's standard commercial practice for all apps".
Ben Thompson of stretechery has been commenting on this recently, mentioning that US antitrust law is based more on how people compete (following rules) vs. EU antitrust, which is based more on consumer outcomes and harm. And the harm in this case seems pretty clear: Apple's behavior, though perhaps unintentionally, is making it very difficult for Spotify to compete with Apple Music -- EVEN THOUGH (in the US regulators' view) they aren't acting with any kind of malice, or illegal intent (their view).
The hard legal question is how to ensure competitive markets when tech platform operators make products that inevitably lead to monopolies, like the App Store. That one really should be legislated because it's a big, complex question that will come up with escalating frequency as more businesses exhibit tech-like winner-take-all dynamics.