I thing the general Idea is, with spotify, people are not buying CDs (or digital files) anymore, because they have the liberty to choose all the time what they wanna hear. So Spotify must compensate for the lose. At the end Spotify is a mix between radio and CDs, but more on the CD-Side, because with Radio you usually can't choose what you hear.

I wonder if Spotify also pays different depending on whether people choose a song or whether it was autplayed because of some public playlist or radio-feed (does spootify have something like that?).