Spotify on iOS exposes metadata and basic commands. When music is playing, the track info and album art shows up on the lock screen, and you can pause or skip using the lock screen buttons. Apple makes a standard API available for that stuff, and apparently Spotify is happy to participate.

Seems to me that Apple could pretty easily let any app opt in to Siri integration too. All they'd really have to do is pass commands saying "play song/album/artist named X" to the app, and let the app handle it from there. Add a setting somewhere to let the user choose which music app they want to use for Siri, or make them specify it as part of the command, and off you go.

If Apple offered this and apps like Spotify refused to participate, then maybe we could talk about this going both ways.