The profit from the sale of one MacBook is equal to that of 250 Snow Leopard discs. There are 14 PCs in use for every Apple computer. Suppose half of all PC owners bought a Snow Leopard DVD, and at the same time, half of Apple's profit from the sales of computers was lost because a lot of customers bought PCs and installed Snow Leopard on them. Do the math and get back to me.
Apple sells the Mac experience based on no-compromise hardware and software developed by the industry's top designers. Would a Ferrari engine installed in a Ford pickup do a lot for the reputation of Ferrari?
You have to realize also that not all PC owners bought Vista or even XP. Many of them used a friend's disc to install. Microsoft doesn't make a penny on that. Here in China you can buy Vista and XP on any street corner for US$1. You can also buy Apple computers, but not cheaply. No one can simply burn a copy of a computer. Every one sold is profitable.
Check the stock performance of Microsoft for the last ten years compared with that of Apple. Which company is leading the industry and which is following? Microsoft doesn't have to put as much development into Windows because half the technology of Windows is taken from OS X.
Last important fact sheet: Apple DID SELL MAC OS to other computer makers from about 1995 to 1997 when Steve Jobs was pushed out, and a "genius" named Gil Amelio was the CEO of Apple. They nearly went out of business because of it. The stock hit a 12 year low at US$3.53 and they lost over 700 million in three months. Steve Jobs quickly put an end to that hair-brained scheme by changing the release version of OS 7.7 to OS 8 (Mac OS licensees had a contract only for OS 7 versions) and doubled their licensing fees for the next proposed contract, thus ending the clone program in a flash of real genius. Only one clone maker, Power Computing, tried to sell computers pre-installed with OS 8, so Apple bought them for 150 million -- with money paid to Apple by Microsoft in an out-of-court settlement about copying Mac OS. I know because I did tech support for Apple at that time, and they took in all the tech support for Power Computing for one year as a courtesy to those Mac users.
Apple now has 34 billion (not a mistake) in cash on hand and their stock is at US$204. I hope they continue to ignore the PC market. You may over-estimate the love that the PC market has for Apple. Right here in this forum I often read comments that OS X is terrible and Windows or Linux is the best. It's the age old Chevy-Ford-Dodge argument my brothers used to have.
Incidentally, Linux is UNIX-like because it did not use any pre-existing version of UNIX as a base. OS X is UNIX, see link below.