the TJ clouiter story goes like this (not sure it was actually him tho...)
He's involved in a pot for a large % of his stack when he realizes that the dealer has taken his cards, he then puts his hands on the table to conceal the place where his cards WOULD BE so it looks like he just covering his cards with his hands.
He then went all in or jammed on someone w/e and everyone folded and he got the pot

The difference here is that its a SHOWDOWN, in order to win at SHOWDOWN, you have to show your cards SIMPLES.
Reasoning for this is as MANTIS says its just an opportunity to police for foul play.
SHOWDOWN = SHOW BOTH CARDS and there are no exceptions.