It's cheaper / more economically efficient for the consumer or buyer to purchase Windows alongside their computer purchase.
Sure, I hate it that you really can't get a PC without an OS, but hey, the average consumer that buys a pre-built PC will want Windows. If you use BSD/Linux, odds are you'll end up getting a hand-me-down / building your own to use it on.
Eh I've seen a complete tower with XP Home installed + OS cd, for 299$ at tigerdirect. XP Home, alone sells for 89$+, and that desktop was worth more then 210$. I beleive it was one of the wintergreen systems they offered way back.
