Many things are technically possible. Mac OS X uses the Carnegie Mellon Mach Kernel with FreeBSD UNIX APIs and many of the FreeBSD UNIX utilities, plus a mix of other Open Source UNIX utilities where their license does not conflict with the Mac OS X license.
You mentioned Linux portability. Linux is developed with portability in mind, where often times companies interested in making Linux work on their platform, provide the porting labor.
Apple is only interested in making sure Mac OS X runs on their hardware. And since Apple is a hardware company, that happens to make great software to sell their hardware. Not other people's hardware.
The Mac OS X license under which Apple sells their software, specifies it is licensed to run on Apple hardware. Not someone else's hardware. So if you load Mac OS X on 3rd party hardware, you are in violation of the Mac OS X license.
You also mention Windows. Microsoft started out as a Software company, and make their revenue based on software sales. Windows does not sell for $20, where as Apple gets its money from hardware sales, so Mac OS X is subsidized by the hardware profits.
Basically you are comparing different business models and profit methods, and trying to cherry pick the features from all three to wonder why Apple does not allow you to run Mac OS X on someone else's cheap hardware.