WTF are you talking about? Every single home user that bought a PC with Windows made a choice to not buy a Mac with OSX and the same the other direction.
To answer your articulate question, we're discussing the two operating systems OS X and Windows. Yes, I'm well aware of the fact that buying a PC involves choice, heck anything does. My point is more to do with Windows. The vast majority of new computers sold in the market today come pre-installed with a version of Windows. It doesn't matter if you already own a windows licence, or intend to run Linux, you have to buy a new one, like it or not.
The keyword being 'vast majority', there's no doubt that building your own PC or some specialist shops could sell you a computer without an OS. But we're talking 95%, which means average Joe. Joe is unaware or doesn't care about a viable alternative OS on the PC. To him, a Dell running on Windows is a computer. Mac's are for creatives and Linux is for geeks. A person who went ahead and bought a mac for personal use, now that is exercising choice because you can't possibly argue that that person has no clue about Windows.
Choice without knowledge is useless. Arguing that there IS choice is absolutely pointless in this discussion because thats true in any instance. If there was a single candidate in an election, you still have a choice... vote for him or don't... but does it mean anything?
A real choice requires understanding of at least some alternatives and being able to make a rational decision based on that knowledge. If you want an example to prove the above, just look at Linux on the desktop. The barrier of entry is insanely low. The cost is free, there are live CD's to try it, the installers are unbelievably easy which automate the dual booting process. but no... I install Suse on my girlfriend's computer and she starts crying saying that it doesn't feel like her computer anymore!