Apple is playing a slight of hand trick here. Mavericks isn't free. Apple makes higher margins than anyone else in the PC hardware industry, and you can only run OS X on Apple hardware (I'm talking broad market here, not hackers). What Apple has done is combined the hardware and software in to a single SKU.
No one gets all excited when a new version of iOS is free because that's just "the way things are" in mobile. I think the strongest reasons for paid OS updates on the Mac in the past are because of precedent. When IBM chose to license DOS from Microsoft, they created the operating system pricing model that would define all future operating system pricing models in the consumer market. Apple has always charged for OS updates because that's just how OS updates were done.
Apple has learned a great lesson from the success of iOS and the app ecosystem there. The consumer doesn't want to buy hardware and operating system software, they want to buy a device. That device should fulfill their other wants and needs, which are delivered through software that have been abstracted through the term "Apps".
A lot of people fear a wholesale re-implementation of OS X as a giant form iOS that runs on a laptop/desktop. I don't think that's where Apple is headed. Rather, Apple is going to apply the same broad strategies to OS X, but will maintain a distinction in the actual implementation (tactics, if you will).