Apples and Oranges. Apple do not let other manufacters sell OSX. If they did, surely you'd see people adding little apps to the preinstall image and undercutting Apple. Apple clearly don't want this, so they disallow it.
Windows and OS X are substitutes, that’s what makes this a fair comparison. You would have a point if Windows wouldn’t even be an option for those who ultimately buy Macs and if OS X wouldn’t even be an option for those who ultimately buy PCs.
Also: What stops PC manufacturers from creating their own OS? (Obviously not from the ground up but they could create a Linux distro ála Ubuntu.)
