It is default. The overall system integrates with it. iOS won't offers up Spotify or encourage a Spotify subscription. Anyone interested in Spotify has to manually make it work.
Same with Maps. Click a location in Calendar and Apple Maps is going to open, not Google Maps. The former is the default too, integrated with the rest of the system... much like IE was back in the IE vs. Netscape days.
My point was that the default (or integrated) system usually wins but look through even this thread and you see plenty of Apple people commenting about favoring Spotify (anyway), or trying AM and switching to Spotify, etc.
Note this is no attack on Apple- just an observation that Spotify appears to be surviving an IE-like default option threaded into the iOS... even with a segment of us Apple people. IMO, that's a good thing as competition is much better for all of us rather than even Apple completely dominating anything such that all competitors completely die out.