It;s more complicated...
Mainfram OS is not "sold" that's the problem. It;s licenced by MIPS per CPU minute. It;s a continual fee, not a fixed price, and is as much a piece of the hardware as RAM or disk. The faster you want the iron to go, the more the OS costs. Splitting the OS price from the hardware is not done, there's simply no valid cost adjustment possible for it. So much of the hardware was designed exclusively for that software, how do you quantify it? I'll tell you; the price would be so high, 30 or 40% of the box if not more, that noone would ever choose to actually pay for it without also getting the hardware. IBM does not have to support the software, but the would have to charge for it. They'd also have to support their OTHER software that IS seperately licensed on it, and that's a whol nother bugger.
Apple chooses not to sell it;s software as well. Nothing illegal about that. They provide an OEM version with each unit and sell upgrades, but they do not sell retail boxed full versions. They've discussed it in the past. Excluding iLife, $400 is the epected retail price. Go on, but comodity hardware, install the OS, and see if you can get the same price/performance ratio: you can't. Now, where apple a merket unto themselves (a place where Dell, HP and others have all fallen to Apple's might) and they continued to do the same, once Microsoft is no more, then their continued action could be seen as an anticompetitive action to the hardware marketplace, and the bundling of the OS with hardware could be seen as anticompetitive to the software market. Fortunately, Apple has less than 10% of the market, and clearly can;t be considdered anticompetitive if there's a thriving market. Such is NOT the case for mainframes, so there very well may be a case forcing IBM to license the hardware or the software for others to resell (but likely not both).