Why exactly is it bad etiquette to ask to see a losing showdown hand?

[pig4bill]

I've used it when the player is supposed to show but won't. There are a few of these types where I play the most. I've called his bet or it was check-check on the river, and his order on the table dictates he show first. The dealer tells him to show but he refuses, and he won't muck either. So I tell the dealer IWTSTH and then show mine.