OS X certainly is a major selling point for Apple, but it is not the only selling point. After all, Apple controls a huge swath of the personal media player market and iPods don't run OS X.
That being said, I agree with you that Apple does not want Psystar - or other OEMs - selling OS X without their permission because it will likely have an impact on their business. But far more dangerous then the impact it will have on the business, it will be the impact on the product that really keeps Apple's lawyers up at night - and should keep you up at night, as well, if you have any serious long-term experience with the Wintel world.
It is the entire experience of Mac ownership that makes it desirable to own. It is easy to buy. It is easy to set-up. It is easy to use. And it is easy to maintain. None of those apply to a Wintel machine and I am not being flippant when I say that.
Replacing Windows for OS X is not going to make the experience of buying, setting up or maintaining a Dell or an HP any easier. And it may not make it any easier to use. Those who believe that if only Apple licensed OS X to major Wintel OEMs it would quickly take a commanding share of the market are not giving that statement the thought it deserves.
And be careful what you wish for. If Wintel OEMs make OS X boxes and Apple only makes money from selling OS X like Microsoft, Apple will have to adopt the same practices Microsoft does. They will need to have OEMs make cheap quality PCs that last 12-18 months so you have to replace them with a new copy of OS X. They will charge many multiples of the current $130 for retail copies of OS X and will imply onerous DRM and authorization/activation/verification schemas like Microsoft does to make sure that you continue to "render unto Steve what is Steve's", to coin a phrase. And as OS X becomes more and more popular, the malware community will start to focus on it with more and more emphasis. Sure, Unix has 30 years of security experience under it's belt, but it also has 30 years of attacks aimed against it and I bet a great deal of them have not been discovered or closed so you will still be vulnerable.
And finally, if OS X is why people will pay more for a computer, why would any OEM want to license it just to make no money? Again, people who believe that if only OEMs could license OS X so they could give us are $599 mini towers and $999 laptops are not giving that statement the thought it deserves. Those OEMs want to be able to charge $1499 for a desktop or $1999 for a laptop too, and if OS X will allow them to do so, they're certainly not going to charge less just because it's the "Christian" thing to do.