How come? Because DRM isn't helping the labels sell music. It's helping a company (Apple) become the music industry's single gateway to the people who want to pay for music online.
Thanks to DRM, a song downloaded at the iTunes Music Store will only play on iTunes or an iPod. That means that if a customer wants to start using different jukebox software or another MP3 player, he'll need to rebuild his music collection from scratch.
As Apple's share of the overall music market grows, it will be more and more difficult for you to walk away from the table during contract negotiations. Jobs will hold all the cards, because his customers--who form an ever-growing share of the music market--will be locked into his products. Like Bill Gates in the PC world, Steve Jobs will become the gatekeeper to tens of millions of music fans, and you will have to pay his price for admission.
How does ditching DRM help? If Apple's songs were distributed without copy protection, your customers would be able to switch to another program at any time. You could threaten to cut a deal with any of the other companies now clamoring for your business--Real, Napster, Sony, Microsoft, etc--and Jobs would know that his customers had the option of leaving his platform.
I know what you're thinking: what about piracy? The reality is that DRM does next to nothing to reduce piracy. Virtually every song ever recorded is already available on peer-to-peer networks. It's easy to "rip" a song from a CD (which has no protection at all), and Apple's DRM scheme has been repeatedly cracked. So people who don't respect the law aren't going to buy songs from the iTunes Music Store in the first place. DRM won't do a thing to stop them!
On the other hand, DRM systems treat your most honest customers like criminals. People who purchase music from the iTunes Music Store know perfectly well that they could get the same song for free via a peer-to-peer network. They choose to purchase from iTunes for one of two reasons: they value convenience or they respect the law. Either way, you don't need DRM to keep them honest. If they were inclined to engage in piracy, they wouldn't have bought the song in the first place.