Pandora is a weird service. They pay less for their music in exchange for putting restrictions on it that make it less likely to substitute for music purchases. It really is supposed to just be radio, and they do things like limit skips so that you can't just skip ahead until you get to the exact song you want to listen to. The music companies want you to still go out and buy the album if you want to do that. On the other hand, Pandora is cheaper, at $5/month versus, for example, $10/month for Spotify.
Personally, I wish I could get Pandora's selection algorithm's into Spotify and have the best of both worlds.
