It's more interesting considering what this means as an Operating System developer. If they're giving away copies, it shows that Operating Systems are truly commoditized (as well as the huge profits that Apple is raking in!)
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).
When you buy an off-the-shelf PC, you have the same experience. The Windows licence is included in the price of the computer, for which the OEM pays Microsoft. However, subsequent major releases of Windows aren’t included in the price. Given the trend, that might change. I can imagine Microsoft making all future consumer OS releases free for those with a valid Windows 8 licence. For enterprises, little is likely to change, as they already pay for seats, not specific OS versions.
Edit: Web apps (Google Docs/Drive, Spotify, Basecamp, Facebook, etc.) would fit the bill as a force for OS commoditization.
Much the same way Microsoft made the browser a loss leader because it made money on the OS, Apple is making the OS a loss leader because it's making money on the hardware.
I think you could show that by having much larger %'s of its user base being up to date and adopting new releases faster, that it cuts costs in other areas and makes it a more attractive platform. Just spitballing here, but less users on old versions should reduce support costs, create a better user experience for more of its users, and entice developers to take advantage of new features, which may further differentiate the platform from competitors.
I'd have to think that creates more long-term value for the company than making $30 bucks per license. And that's not even calculating the customer goodwill by making the updates free.
What they are doing here is to integrate ongoing support for OS upgrades into the cost of the computer.