While true, it's irrelevant for two reasons:
1) The radio doesn't play the songs you want on demand; and
2) The radio is effectively a way to drive album sales. Spotify is a REPLACEMENT for the album.
It used to be that it was worth it to play your songs on the radio (even at a loss) because people that liked your one song might want to hear the 10 hours you wrote--that would never be heard on the radio--and spend $10 on the album.
Now you pay Spotify $10/month for unlimited access to the entire album. To the entirety of the artist's catalogue. To the entirety of all the included artists' catalogues.
This is obviously and trivially less money than any one of those artists would make previously from you if you liked their music. Perhaps the argument could be made that more people are listening and giving a tiny amount of money to each artist, but I rather think that given the stats I've seen, this isn't even close to true.
This is much different from the time when people were pirating albums, since many fans would go out and buy an album that they downloaded because they wanted to support the artist. Now people feel that because they're paying $10 to Spotify or Rdio that they ARE supporting the artist. They're not going to pay for a subscription AND an album. That's exactly the opposite of the point of these services.
They need a new model. Streaming on its own for $10/month is clearly not enough money to go around. Spotify has infrastructure costs and has been bleeding money (I think they had a break-even or profitable quarter just recently?). Meanwhile, they also need to distribute the remainder of the already paltry $10 between a zillion artists. It makes no sense.