Do You Care About Apple EULAs?
While Apple doesn't encourage it, the Snow Leopard upgrade disk can be used to upgrade machines that have Mac OS 10.4 Tiger installed as well. However, this is actually forbidden by the accompanying EULA, which raises an interesting question: do you have any moral problems ignoring said EULA? And on a more general note, do you actually care about any EULA at all?
When it comes to Mac OS X, Apple has always had the luxury of not really having to care about piracy. Apple makes most of its money via hardware, and since Mac OS X only runs on Apple machines (technically), even if you do pirate your copy, you already gave Apple the big bucks when you bought your Mac.
The end result of this is that Apple never had to put any Genuine Advantage stuff or serial key mechanisms into its client operating systems. Windows, on the other hand, does have this stuff as an integral part of the operating system, since Microsoft doesn't have a hardware division to fall back on.
