Stuart,
I would have to say that I think you are being a little too "technical," in the situation described above. Mooredog mentioned that the player that displayed one card (showing three of a kind), was a new player. I don't blame him for going balistic. I think a warning would have been more appropriate, for a first offence. The dealer should have intercepted the card before it hit the muck. I think we need to recognize when a player is making an honest mistake, and when something is suspect.
I guess, if a seasoned player were to "show one" card only, then your strict rull enforcement could be considered. I really believe the rule is for players that show one card, and then show the other if there is a need.
You run a "tight ship" and I know that's probably the right way to operate. I do, however, lean toward protecting new players from having the "pot" taken from them when they have the best hand.