Apple is a systems company. Operating System software is only a part of what they do.
The OS is technically available to anyone who buys a copy of it over the counter - it just won't work easily in anything other than Apple's hardware. Constraining the supported configuration insures a consistent level of quality and stability as well as lowers support costs, though it does raise the irritation levels of those who would like to try it, but not enough to buy one of their systems.
There have been "enterprising" folks who have "messed with it" to work on hardware designed to run MS Windows, but it it neither encouraged nor supported. Prior to this issue was the issue of allowing Macintosh clones. That did not work out too well for IBM did it???
The Linux OS is essentially supported by the tinkerers, but it is not nearly as polished, but it does have a LOT in common with the core of MacOS X. In fact, before the MacOS X was released to the masses, early developers were encourages to obtain one of the versions of Linux for PowerPC (MkLinux or LinuxPPC) to get acquainted with its features, and differences from the existing OS version. There was also a version of the OS core called Open Darwin that ran on x86 hardware. Now, the reason for that work was clear.
Originally Posted by prerich
Now you've just hit the nail on the head - Apple should make its OS available to everyone. PC builders like myself love to mess with the inards of our rigs (just as some high end people love to modify the caps and such in speakers and D to A converters).