Pandora has a different model than Spotify, and comparing them is somewhat silly.
Pandora pays the standard rate, set by the government. It cannot play a specific song you request, or play an entire album. Much like traditional radio, you hear what "the DJ" decides to play. You can "choose a station" by selecting an artist or song, but you can't force it to play the song you want.
Spotify, on the other hand, deals directly with the artist, and pays more for allowing to play an album, or specific songs.
At at the end, it doesn't really matter how much they pay the label, because the artists doesn't get shit.
As a side note, the royalties, like in radio, are paid to the song writer, not the performer.