I have a question about Unix and Linux and their licenses.

If you choose to make an operating system based on the Linux kernel then you have to distribute it for free under the GPL License, but if you choose to make an OS based on the Unix kernel (example: an OS based on FreeBSD) do you have permission to make it closed-source and to take copyrights making it a proprietary software distributing it non-free? 

So if somebody chose to make an OS based on FreeBSD can they sell it as their own modified version, taking copyrights or something like that? This question arose because I know that Mac OS X is based on FreeBSD and it should have used FreeBSD licenses, and OS X is a non-free, closed-source proprietary software. 

So, can you do that with Unix? Or does Apple have some sort of "agreement"?