Just to clarify something that hasn't been brought to fore.... Although this particular rule is there technically to prevent collusive behavior, allowing other players at the table to ask to see any hand, it is more accurate to say that the rule is there to deter players from being unsportsmanlike and forcing their opponent to showdown there hand when they are conceding the pot.
In Christine's example Player A has bet and Player B called, with Player A mucking. In this situation Player B is the "calling" player and would like to know what Player A is betting with.
The opposite situation is Player A has bet and Player B called, Player A showsdown and Player B mucks. Being the bettor, aggressor, it is unsportsmanlike to ask to see what Player B is calling with, when they have conceded the pot. This is the situation that brought the rule to state, "If the winner, asks to see the conceding player's hand, then the hand is live".
Because truthfully if a player wants to concede the pot to you by folding, uncontested, and you want to see their hand, you DESERVE to lose the pot if their hand is better than yours. You have to ask yourself what is more important - The Chips? or the Information?