It's legacy.

The FAA has a distinction between commercial and recreational flying. When the laws were written, commercial meant that you were carrying passengers in a full-sized aircraft. And it makes sense to have more rigid rules in place for passenger aircraft.

Times have changed and using commercial vs recreational doesn't make sense anymore. The FAA is trying to apply existing laws to new technology. So, we're left with the case that it's perfectly legal to put a camera on a toy RC aircraft and take pictures for fun. But if you sell those pictures now you've crossed into commercial territory and your flight was no longer legal.

edit The good news is that the FAA is working on new rules. They're just miserably slow about it.

Just a few weeks ago the FAA released its roadmap for new UAV regulation where they discuss some of their plans for regulating commercial activity: http://www.faa.gov/about/initiatives/uas/media/uas_roadmap_2...

Lots of people have been waiting years for this, and are still waiting for more detail, from drone startups (check AngelList to see how many have started in the past year) to guys wanting to sell aerial photography services.

Probably for similar reasons why you need a commercial pilot license. If you're going to be doing it for money, you should have more training than someone just going to wal-mart and picking up a drone. However, as such a training regimen has not been defined yet, the FAA has decided that it's safer just to say it's off limits for now.

Personally, I'd love to see more effort into setting up such a commercial program, as there are a lot of dangerous jobs that could be made safer through drones. For example, there are dozens of crop duster accidents each year where someone's injured or killed. Pulling someone out of the pilot seat, and onto the ground can make that job a lot safer and more accurate. But, we can't get there until we do the studies to figure out how to do such safely.