Declaring an emergency seems to absolve you from a lot of potential
legal problems since the aviation regulations in most countries permit
any reasonable action to save the aircraft and persons on board.

There doesn't seem to be any strict legal test for what constitutes an
emergency. If the pilot thinks he has one, that's enough in most
cases. Inability to find lift on a cross-country flight is pretty much
akin to engine failure of an airplane and could constitute an
emergency.

This should also be the case with gliders landing in US National Parks.