My understanding of the current philosophy behind the law is that if you create a market for organs, you create the opportunity for various abuses such as organ theft.
You are certainly free to donate your organs, before or after you die. However, the state and federal government does have the power to regulate commerce, and can (and does) prohibit commerce that is not in the public interest.
Tony Soprano: You know, we’re the only country in the world where the pursuit of happiness is guaranteed in writing… Buncha fucking spoiled brats. Where’s my happiness then? Dr. Melfi: It’s the pursuit that’s guaranteed. Tony Soprano Yeah… always a fucking loophole.
One problem that hasn’t been adequately explored is ability to benefit. Organs are allocated based on ability to benefit. Joe Bucks might die on the table, or 2 months later, while poor Bob might have gotten 10 productive years out of the same organ, but he died waiting. Some mercenaries might not care, so long as they, or their families got the check, but some people do.