The main OSX desktop is subsidized by hardware sales. That just means it gets to extends to other platforms and products. Hardware doesn't have to subsidize one thing. Apple's hardware sales allow it to develop and maintain OSX which is not only used on its main hardware but serves as a core for other platforms and products. I see nothing wrong and immoral about that.
You're correct in that Apple's hardware sales allow it to continue to develop OS X (along with other products), because that's how most corporations work. Corporation Alpha brings in X amount of cash. It has to spend Y amount on development of its various products. Hopefully X is greater than Y, but they can apply X however they wish towards Y.
However, for legal reasons, Apple cannot claim that OS X's cost is subsidized by hardware sales. They can say that hardware revenue helps to continue OS X development, but that's a different idea/situation.
Nothing as far as OSX is concerned is "fully paid for" since OSX is an evolving platform that is continuously developed and improved (via bug fixes, updates, enhancements etc). All the consumer ever pays for is physical hardware. The software is never "paid for" its just licensed for a fee. That's why you buy dot 0 releases - software updates are only covered by the license up to a point. Thats the advantage of intelectual property, you can bundle it, and distribute it an any for, you want to.
The consumer pays for a license. Apple could charge you for the 10.6.1 license, for the 10.6.2 license, etc., but they don't. They (currently) only charge for major revision (i.e. 10.X) licenses.
However, that having been said, they can't subsidize the license cost via hardware. Revenue (be it from iPhones, iPods, Mac hardware, etc.) contributes towards OS X's development costs. But in regards to OS X's licensing costs, Apple can charge whatever they want, be it the necessary price to recoup development cost, or at a price that ensures many more people will purchase it (albeit potentially resulting in a loss relative to development costs).
There's also the fact that far more Mac users will buy an OS X upgrade/installation at $29 vs. $129. At $129, I'd be willing to bet that OS X piracy would be fairly high.