Voluntarily showing hands after everyone has folded

[B4TTERY]

Here's a situation from a home game. All but me and the player to my left folded after the flop because I showed strength early and on the flop. The player I was heads up with call my flop and turn bet. After my river bet he folded. Before I tossed my cards in, I showed him my cards to show him he did the right thing (he's not a very good player and I always feel kind of sorry winning all his money). Several people at the table then demanded that they see my hand. I argued that when everybody folds, the winner doesn't have to show his hand. So therefore, if he selectively shows it to some people and not others, it's his right to do so. Somebody brought up the point that it shouldn't be allowed because collusion could be occurring. I didn't really care one way or the other, and willingly showed everyone the hand, but it got me thinking what the general rule is here. What if this occurred in a casino...would it cause a problem? Anybody have any thoughts?