Lots of people getting mixed up between cash games and tourneys here. in a tourney if it goes check check after the river both hands should be shown, in a cash game its not neccesary.
As far as I am aware pretty much everywhere in the world you can muck your hand in a cash game if you want to and it is always considered bad ettiquette to ask to see the losers hand if he is trying to muck it. I have witnessed many arguments over this as it really is nothing more than a rubdown imo. I have witnessed people not in the pot asking to see a hand and it being shown when they have in fact tried to pass the winner and they have then been awarded the pot as the cards were still live. I have also seen many people table what they think is a losing hand to be informed its actually the winner. If anybody at the table notices a mistake being made and the cards are tabled they should speak up- its again an ettiquette/duty to the game situation.
...but this misses a parallel point I made - WHY should it be different in Tourneys & Cash Games?
why does rugby league and rugby union have different rules? they are both rugby after all.
cash and tourney poker is completely different in so many ways so need different rules to govern them.