I had no clue what you were referring to until I read the Snopes, etc., stuff, and still had no clue until you posted your answer. Back when I was a kid, they represented bullies stealing the little kids' shoes to torment them. I haven't seen any for years; none in my current town, and none in the town where I'm moving. I assumed it was because hazing has been pretty much discredited.
What they represented in the present context is that with the barefoot pedal, one must have bare feet. Jinman, I'd like to see the contortions one must go through to use the right hand pedal... /forums/images/graemlins/crazy.gif...the one that makes you go backwards should be the upside down one (maybe it is, I've never drive a 2-pedal tractor), so when you turn upside to use it, you'll be in position to see behind the tractor.