18 vs 21: Moronic Idiocy in the Law

If you've ever looked at the cover of an adult magazine in the store behind the counter, you've seen the obligatory "All Models are at least 18 years of age" or some such at the bottom of some of the more obscure titles. This means that no minors were photographed nude, or while engaged in sexual acts, and paid for it.

If you look further, a sign is posted on the rack containing the magazines or videos (or on the door of an adult book store or bar) that says, "NO ONE UNDER 21 ALLOWED," or "MUST BE 21 OR OLDER TO PURCHASE."

So, let me get this straight: If you're 18, you can take off all your clothes and pose for pictures, or have sex in front of a camera and get paid for it, legally, but you must be 21 or older to be able to legally buy and view what you were legally paid to do...?!