Non sequitur much?
That's not what it says on the package, outside or inside. You *can* install it as an upgrade on a system currently running Leopard (or Tiger, for that matter, since the earliest MacBooks and MacBook Pros predate Leopard). Leopard being installed is not a prerequisite, explicit or implied.
The statement "Upgrade from Mac OS X Leopard with Snow Leopard [...]" is descriptive, not prescriptive.
Statements on Apple's online store, or in their brick and...