The same is true with just about every other electronics device. I might love the design of the Moto RAZR V3, but hate the software. I might want to install the Sony Ericsson or Nokia UI in the RAZR, but this is not an option and neither do I feel that my choices as a consumer are limited because of this.
When Apple decided not to OEM the OS, they made it rather plain that Apple is about the total user experience. Jobs has gone as far as to say that Apple is not about making the most obscene amount of money as quickly as possible. To be sure, they will grow their market with the Mac mini and other entry-level machines, but they will do so while remaining true to their ideals.
The iPod, iTunes, and iTunes Music Store integration is often cited by Jobs as an example of delivering a complete user experience. I, for one, do NOT want to see Apple commoditize their operating system.