RPM is licensed under the GNU General Public License, or as it's more commonly called, the GPL. If you're not familiar with the GPL, it would be worthwhile to spend a few minutes looking it over. The purpose behind the GPL is to ensure that GPL'ed software remains freely available.
"Freely available" doesn't necessarily mean "at no cost," although GPL'ed software is often available by anonymous FTP. The idea behind the GPL is to make it impossible for anyone to take GPL'ed code, and make it proprietary. But enough preliminaries! The best way to understand the GPL is to read it: