Umair Haque explains why. An excerpt:
Here's a final, "strategic" point: every time the music industry kills an underground distribution channel, a more efficient one arises in its place. Goodbye mixtapes, hello www. Bye www, hello Napster. Bye Napster, hi BitTorrent. Bye BitTorrent, hi anonymous, ciphered, totally decentralized p2p nets.
Why? By limiting the supply of interaction, the music industry is only ensuring that each interaction becomes more and more efficient. The endgame is a distribution system where every song in the world in the world can be zapped invisibly and anonymously from me to you in a nanosecond.
This is why all mass-market digital media will end up being free, because it is going to be shared. In fact, the value is in the relationships created over the media, which can lead to monetary exchanges. Whoever intermediates those monetary exchanges wins.