Yes, Mac OS core is based on BSD, a unix-like OS, but that is just one part of the whole OS and a lot of the code in the core was actually written by Apple. The fact that it has a POSIX interface does not make it a Linux. And as for the hardware, would you put Windows and Linux guys in the same bag just because they run their OS on the same hardware?
Well, to be fair, a good chunk of Darwin is taken directly from FreeBSD (most of the userland tools), and a good chunk of xnu was written by what was (at the time) NeXT. That said, most of the mid-level stuff -- the APIs such as Core(Data|Image|Audio|Video|Animation), a good chunk of Cocoa, etc. -- was indeed written by Apple.
Still, it's not as though Apple wrote the entire OS. There *is* a very large amount of open source code that they didn't write. (Examples: CUPS, Apache, the BSD userland, SQLite, OpenSSH, X11, gcc, PHP, and many more.)
And they are still putting it all together so tying the OS to the hardware is their right. What matters for most customers is the final product. They don't care if there is Intel or PowerPC in it as long as it works like they want.
Well... it's their "right" to introduce that sort of artificial restriction, sure... but I'm not so sure it's their "right" to try to punish people who wish to circumvent said restriction. That's a debate for another time though...
Need help with Linux or FreeBSD? Catch me on IRC: I'm ThinkRob on FreeNode and EFnet.
Current laptop: Lenovo ThinkPad T420 (running Debian Wheezy) - kestrel [pending retirement]
Current workstation: IBM Intellistation 9228 (running FreeBSD 9.1) - blackbird