Did you know that it's a federal crime to listen to your music on the wrong brand of portable music player? It's true. At least, it is if you buy your music online. Take Apple's iTunes Music Store, which recently sold its billionth song. Apple talks a lot about the convenience and affordability of its music, but they don't mention its darker side: It's only compatible with Apple products. MP3 players from other companies aren't supported.

In a free market, that wouldn't be so bad. Manufacturers of competing MP3 players, such as Sony, Creative and Samsung, could provide software to automatically convert the music to a compatible format. But that would be illegal, thanks to a little-known law called the Digital Millenium Copyright Act, which Congress passed in 1998.

The DMCA was billed as an anti-piracy measure. It prohibits anyone from "circumventing" a copy protection scheme such as that used to scramble songs from the iTunes store, or from creating software to do so. It was hoped that those restrictions would prevent hackers from unscrambling music or movies and uploading them to the Internet.

But that's not how things have worked out in the real world. After nearly a decade on the books, it's hard to find any evidence that the DMCA has reduced piracy. More and more content is being released in copy-protected formats, but as every college student knows, the latest movies and songs are still widely available on peer-to-peer file-sharing networks. Even worse, new copy-protection formats are routinely hacked within weeks of release, and hacking tools are readily available from offshore Web sites. "Copy protection" hasn't done much to prevent copying.

Most computer scientists think that's not going to change. Indeed, Apple CEO Steve Jobs has said as much. In a candid Rolling Stone interview shortly after the launch of the iTunes Music Store, he said that "we have Ph.D.'s here, that know the stuff cold, and we don't believe it's possible to protect digital content."

But if Jobs knows that copy-protection technologies don't stop piracy, why does Apple use them? One reason is that the recording industry, whose executives lack computer science degrees, has insisted on it. But another reason might be that it allows Apple to lock its customers into using its products. Under the DMCA, software to convert your copy-protected music library to a different format is illegal. That means if a customer wants to switch to a non-Apple device, he will need to re-purchase all of his music in the new format.

Such compatibility problems aren't limited to Apple products. Customers who buy music from Yahoo's online store — which uses Microsoft's copy-protection software — have the opposite problem: They cannot listen to their music on an iPod. Ironically, Microsoft has adopted the slogan "Plays for Sure" for its format, despite the fact that it's not compatible with the world's most popular MP3 player.

Nor is the problem limited to music. Video technologies are an even bigger mess. Google sells videos in a format that is incompatible with virtually all portable video devices on the market. Videos purchased from the iTunes store — like songs — only work with Apple products. Perhaps worst of all, there is no legal way for DVD owners to watch their movies on portable devices like the iPod. It's legal to "rip" your CDs so you can listen to them on your iPod, but doing the same thing with your DVDs is against the law.

Congress may have intended to target copyright infringers, but in practice the DMCA mostly harms paying customers by preventing them from playing legally acquired content on the device of their choice. The Constitution says that the purpose of intellectual property is to "promote the progress of science and the useful arts." When a copyright law begins to interfere with people's freedom to enjoy the content they have legally purchased, Congress needs to give it another look.

Timothy B. Lee, a policy analyst at the Show-Me Institute, is the author of the new Cato Policy Analysis "Circumventing Competition: The Perverse Consequences of the Digital Millennium Copyright Act."