Not a monopoly
As others have stated.
As far as I'm aware, Apple don't specifically market their OS separately from their hardware. The TV ads all talk about 'Mac', and the OS is called Mac OS X. All the implication is that you buy the hardware and it happens to come with OS X... As others have stated, much like you buy a Nokia Mobile phone and it happens to come with a Nokia firmware/OS
I think the car analogy is ok (though not brilliant), as Mercedes have chosen to sell their car engines to other manufacturers who have asked to buy it. Mercedes would be within their rights to refuse, but in that case I assume they have decided that it's in their business interests to sell the engines to third parties. Apple have decided the other way, which they're perfectly entitled to do.