From Joel on Software
The answer is that pricing sends a signal. People have come to believe that “you get what you pay for.” If you lowered the price of a movie, people would immediately infer from the low price that it's a crappy movie and they wouldn't go see it. If you had different prices for movies, the $4 movies would have a lot less customers than they get anyway. The entertainment industry has to maintain a straight face and tell you that Gigli or Battlefield Earth are every bit as valuable as Wedding Crashers or Star Wars or nobody will go see them.
Now, the reason the music recording industry wants different prices has nothing to do with making a premium on the best songs. What they really want is a system they can manipulate to send signals about what songs are worth, and thus what songs you should buy. I assure you that when really bad songs come out, as long as they're new and the recording industry wants to promote those songs, they'll charge the full $2.49 or whatever it is to send a fake signal that the songs are better than they really are. It's the same reason we've had to put up with crappy radio for the last few decades: the music industry promotes what they want to promote, whether it's good or bad, and the main reason they want to promote something is because that's a bargaining chip they can use in their negotiations with artists.
And Apple? Apple wants the signaling to come from what they promote on the front page of the iTunes Music Store. In the battle between Apple and the recording industry over who gets to manipulate what songs you buy, Apple (like movie theaters) is going to be in favor of fixed prices, while the recording industry is going to want variable prices.