People see an open elevator door, they get in the elevator. They don't stop to read the instructions because they are not expecting the elevator to come with instructions and also because they know the elevator doors are not going to remain open forever.
These people are already getting on wrong elevators. The example given has banks of elevators for different floors, marked by sign and by buttons for the specific floors covered. If they don't pay attention they'll get a short tour.
If this was a deadly choice, between an elevator and a trash compactor, it might make sense to over-sign it. As is, it's a trivial and obvious mistake, especially if each bank has a sign saying 'Floors 1-x are reached via the other elevator bank' for someone to read on their round-trip.
Part of the reason users are so clueless is because we go too far in padding their environment. People don't slice off fingers because the sharp end of the knife wasn't properly labelled - they exercise caution.
