If you ride the MUNI buses, you probably already know that to open the back door, you need to step down into the stairwell, and the door will open automatically. That's not really the most intuitive way for things to work, but whatevs, that's cool.
Here's the problem. Let's say you've got someone who doesn't usually take the bus. They pull the bell on a string, ding ding, . Bus stops. Our newbie commuter, obediently reading the signs in the bus that say "EXIT THROUGH REAR DOOR," moves to the back and makes to walk down the steps. But no! There's a huge sticker on the door that says "DO NOT STAND IN STAIRWELL." What are the newbie commuters to do? They blink confusedly. The bus driver starts to pull away. The commuter starts screaming, "Back door. Back door! BACK DOOR!!!!!" Meanwhile, other helpful passengers start shouting, "Step down. Step down! STEP DOWN!" The stress, the confusion, the anger!
If you're a seasoned commuter, you already know that the instructions for opening the back door are printed on the bus -- over the stairwell itself. But who's looking up over their heads when they're trying to get out of a bus? No, they're looking at the window, desperately trying to tell if they've already passed the cross street they want. Would it kill MUNI to put a "Step down to open back door" sticker on the actual back door window? Presumably, it could take the place of the "do not stand in stairwell" sign, since -- of course -- the reason why you can't stand in the stairwell is because when you stand in the stairwell, you..... open the back door.