If it were labeled something like, "novelty product, not to be used for actual navigation", I could see that, but GPS manufacturers position their systems as ones you should actually use for navigation.
-- This is a great point.
I'd almost prefer this to having to click the damn button/s evertime. =D It would also reduce the dissapointment when finding out how crap many (even expensive) maps are in the field. I know know to cross reference (especially topo's), pick the best ones, and cary a spare to diversify just in case.
Perhaps a lawyer will chime in, but on the technical point: its pretty common for people selling product X to limit their exposure to $=price of X (max). So this technique (a limitation of liability resulting from <use> of the product beyond replacement) is fairly common. I don't think magellan wants to get sued when your galleon of gold bullion gets lost at sea...etc. That kind of thing.
GPS navigation devices and software could be marketed "geography teaching tools" or something like that "not for navigation"