Wouldn't it short hot and ground then, and still turn the necklace into a short-lived fuse?

The more practical reason to mount ground down is that wall warts with ground pins or polarized prongs nearly universally arrange them so that they're hanging down when inserted into a ground-down plug. If the plug's flipped, the wall wart's upside down and its weight is trying to lever it out of the wall.

Yes, in that case it would short hot and ground, which is effectively the same and hot and neutral, since at the main panel hot and ground are bounded together. But if it were, say, a metal credit card or something rigid, it might just fall on the ground, or could hit the ground and neutral.