There are artists that Spotify has different deals with. Spotify promotes their music in their playlists, but the artists get a much smaller cut of the profits in exchange. Win-win for everybody.

This only happens in genres where most listeners don't care about the artists they're listening to, think "chillout", "focus" or "easy listening." That kind of music is a commodity, Taylor Swift (or Metallica or Mozzart or whatever) is not. This has been proven.

My hypothesis is that those genres would otherwise lose Spotify the most money, as people often play that kind of music and never turn it off. Because Spotify pays per listen, the user who attentively listens to their favorite artist a few times a week is much better for them than somebody who has "chillout" playing on their echo 24/7.