Does this have anything to do with 'upper deckers'?

Good grief...

Often "do not" signs just give people ideas - if it's a thing, why not try it just to see why someone would - they might think. Pretty sure I'd fall in if I tried it.

if someone is going to do that a sign is not going to stop them. To me the person is trying to hide so they are not going to obey the sign.

Im not sure why that sign would be in Missouri but from what I understand you see it a lot in pacific neighboring regions because a lot of mainland china uses a hole in the ground, squat toilets. if your region has a lot of tourists from that area sometimes they don't know what else to do with a toilet lol.