The Mac OS X license only allows Mac OS X to be run on Apple Macintosh computers.
Any company actually selling generic PC systems which include Mac OS X would be operating illegally.
It is technically possible to build a PC that includes identical components to a Mac, and with some software hacking, get Mac OS X to run. But that would still be a violation of the Mac OS X Software License.
Also such as system is often difficult to update, as each new release of Mac OS X may require new software hacks to get running on the generic hardware.
And Apple will not provide any support for your system, and these forums will not provide user-to-user support as that would be a violation of the forum agreement you agreed to when you signed up for an account to this forum.
That website shows a real Mac Pro installed in another enclosure:
... The computer is a Mac Pro Quad Core 3.0 GHz Intel Xeon Dual Core 4GB Ram 500GB hard drive Desktop. ... All machines work just as they would in their original states as only the console cases and power sources have been changed.
There is probably nothing wrong with that.
It's a lot different than installing OS X in non-Apple hardware as your question implies.