I guess the disk and the box have become collector's items. How much is my copy of Photoshop 1 worth?
Anyway, my understanding was always that if you bought a newer mac it could not run an older system, only the one it shipped with or newer.
Not at all... It's just that historically (there's that word, again), Apple would have stopped selling Snowy discs long ago, and other than some savvy folks, people think eBay is their only avenue. (Apple is still selling the discs for the benefit of users who need it to access the App Store to get Lion and later.)
Edit: As a matter of fact, the only way to navigate to the Snowy disc on Apple's Website is to follow the "Upgrade to Mavericks" links.
Generally, a Mac can't run a version of OS X that's more than a tick or so older than the version with which it shipped, but tacit recently reported having bought a new Mac that shipped with Mavericks but could run any version of OS X going back to Snowy.
