Well I can't speak for the GP but I'd like to respond to yours.
Any purchased disk of OS X has no serial verification or copy protection. All allow a fresh install as far as I know. Now the fact that they do this (and don't try to restrict the number of machines which it is placed on) is, I believe, separate from the licence agreement. The offer of a cheaper upgrade for Leopard users was meant in some way as a nod towards the short time between Leopard coming out and Snow Leopard. Somebody wishing to move to Leopard from Tiger was expected to pay the full amount, just as they are expected to if you move from Tiger to Snow Leopard. There very well may be perfect examples of Apple doing as you suggest, I would argue that this isn't one of them though.