Apparently its something about Apple making most of its profits from the sale of its hardware, over operating system - eventhough the operating system is more advanced than Windows and Linux.
Basically, if you want the Mac OS, you have to shell out a couple of hundred quid for the Apple box it comes in - and I don't mean cardboard!
If Apple sold its OS as an user-installable item, Mac sales would most likely plummit. People could get the worlds most advanced operating system, and run it on cheap, shoddy hardware. Most loyal/power users would go the route of the Mac - and since prices are coming down compared to what they used to be (in some respects cheaper than budget-ware Dell) - you would still probably get some people who buy hardware and all.
But for most users wanting to explore the 'dark side' - they could just buy a disk, and Apple would lose millions.