There's something about the mac community that makes them more willing to pay for their software than the windows or linux communities.
In windows, I think there is more theft. Mac people understand that the only way to keep their platform alive is to cough up the money. That's not true in windows. With linux, the expectation is that software should be free. People will commonly choose a free version even if it's not as nice to use.
Apple will serialize their OS when people stop paying for it.
Vanguard
PS On a seperate topic, I bet that about 90% of their OS upgrades will happen in the first week. After that, it will either come with the machine or people won't buy it. That's different from windows where the upgrades trickle in for years.