As for the "Macs are more expensive" myth, you should read this article from Tomshardware.com http://www.tomshardware.com/reviews/apple-mac-leopard-windows-vista,1985.html
To summarize, the author compared a Macbook to a Dell with the same specifications. The Dell turned out to be more expensive by a couple hundred dollars.
Saying that Apple makes expensive computers is like saying BMW makes expensive cars. Really, they make high-end products and don't appeal to a market looking for low-end "cheapos". Actually, Apple doesn't seem to make any attempt to appeal to the cheap computer market.
The stability offered by making the HW and SW appeals to graphic and video editing professionals. You may find a few that like Windows, but I think you will find most go with Mac. Also, a lot of highly knowledgeable techs like macs.
It seems that Apple has found their market and likes it. Apple does not seem to competing with Microsoft directly, but has chosen to make money where they are good at. Apple is still around, so they must be doing a good job. I would think that Apple has no real interest in being "#1", but rather in making profits. (I say this despite the Mac vs PC commercials)
This is getting a little long, so I will try to wrap it up.
Microsoft did allow any hardware to use their OS. You can, of course, run Windows on a Mac. This means that there is a variety of hardware available for PCs, and this is a good thing. This is what keeps them in business. MacOSX, of course, will only run on Apple computers. This has its own benifits and is what keeps Apple in business. I, personally, believe that these differences brings more options to us as users to choose what meets our needs. However, if you make the decision based on "this company is stupid/evil/greedy", I would have to call you unwise.
As for application support, it is up to program developers to make their software available to Mac. Apple is not stopping them. I don't think Apple could stop them if they wanted to.
To sum up, I like all three OS. Windows is nice because of the widespread use and application support. Apple is nice because it is stable. Linux is nice because it is free and very well designed. Some things are easier in one OS, while other things may be easier in another OS. Also, I believe that Windows is most popular simply because the average person doesn't do enough research to realize that there are better options available. They just go with Windows because "everyone else is using it".
