Linux is licensed under the GPL, which is described as "copyleft." The GPL requires that if you want to use GPL code you need to license your modified code under the GPL.

FreeBSD is licensed under the BSD license, which is a permissive license. Basically as long as you stick the license statement in your documentation you can do whatever you want with BSD-licensed code. This is why commercial uses (like the Wii's OS) tend to be BSD-based rather than Linux-based.