Ad supported streams in Spotify are counted in a separate pool, and only get paid out of the ad revenue pool.

Artists can of course complain that "they're selling our music for cheap!", especially in the ad pool. But what's worth remembering is that when it comes to setting optimal price points, Spotify's interest is almost perfectly aligned with the artists. And Spotify has a hell of a lot more data than artists (not to mention financial sense, which you probably didn't become an artist if you had a lot of).