Sure they are. They can take out ads all over the web, TV, radio, skywriting, whatever. They can't advertise the option within the ecosystem Apple provides, much like I'm not allowed to stick a sign advertising my crap on your front lawn.
Since it's proprietary garbage I'm sure you don't own any of it, so none of this should matter to you (unless you're a Spotify employee).
(No competition on iOS, huh? Google, Amazon, Spotify, Tidal, Apple Music stores. Netflix, HBO, Apple, etc. video apps. Google, Apple, Kindle bookstores. Nope, no competition at all.