Hello Steven,
I think the reason for the repeated error is the procedure that we use for cash games is different. Most dealers deal cash games as well as tournaments. Whenever the showdown is reached, we should look to the side pot winners before the pot that the all-in player or player's are contesting. In cash games, when an all-in player sees an opponents hand that has theirs beat...they can no longer compete for the main pot...that's it, so the act of not showing, or folding is the right thing to do. This is not allowed for tournaments that follow the TDA Rules.
There are many threads on this forum that discuss Face Up for All-in's. Our rules insist that all cards be tabled by all active player's whenever any player is all in. Convincing the dealers to recognize the difference between cash games and tournaments is the only solution. The all-in button would help, but I guess the dealer's just need to be better trained...and yes, if player's insist on attempting to muck, they must be warned and then penalized.