Except that Apple does sell OS X separately from hardware:
Apple Snow Leopard Mac Box Set
Whether I am "underpaying" or "overpaying" when I buy this $169 box set is just your speculation. If Apple wants to cross-subsidise MacOS with hopes of selling more Mac hardware - it's just their business decision. Buying MacOS and using it on my own hardware doesn't make me a "pirate". No more so than buying heavily subsidized Sony PS3 (because Sony hopes I buy bunch of games), but never buying any and just using it as a Blu-ray player
The difference is, Apple sells the OS X box set with the explicit requirement that is be used on an Apple computer.
Apple: Hey you, look at this. I've got this beautiful OS for sale.
Guy: Looks nice! How much?
Apple: If you've already got Leopard it's just a $29 upgrade.
Guy: Great price, but I don't have Leopard.
Apple: That's OK, we also offer it as part of the $169 box set.
Guy: That's pretty steep, but it's a great OS.
Apple: But the deal is, you can only install it on Apple hardware.
Guy: But I just want to install it on this crappy netbook I got from Dell.
Apple: Sorry, we don't sell it like that.
Guy: Why the h*** not?
Apple: Well, look, we have our reasons. But you can probably guess, since we almost went out of business in the nineties trying to sell like that.
Guy: Oh, OK. Well, in that case, I'll buy the box set.
Apple: Er, OK. Just remember, the deal is, you've got to install it on Apple hardware.
Guy: Yeah, right, whatever.
Apple: It's no big deal -- you don't have to buy it if you don't agree.
Guy: No, I agree.
Apple: Are you sure?
Guy: Yeah, I promise.
Apple: OK, here you go. $169 please.
Guy: Thanks... sucka. <runs off to install OS X on a crappy netbook>
Are you that guy?