The simple explanation is that, legally speaking, "do whatever you want with it" does not necessarily entitle you to do whatever you want with it.

In addition to what Cushman said, find out the results of Dr. Richard Hipp (sqlite creater) about putting his software in the public domain. Some countries don't have a similar concept, some companies need explicit permission, etc. It would be easier if they simply chose a very permissive license to use.