"please"

Around campus there are many "garbage centers", if you will, where one will find numerous garbage cans in which to dispose of their waste materials. The cans are separated into different types of recyclables and pure garbage. Often there will be a sign above one of the cans, the paper/cardboard can, asking that no milk cartons be thrown in along with the other paper products. I noticed at one of the garbage centers, the message on the sign is preceded by " 'please' ". " 'Please' no milk cartons", or something to that affect.
Why the quotation marks around "please"? Is this like when someone uses air-quotes, suggesting that they do not really mean what they are saying as it would otherwise seem without the help of the air-quotes? Are they being sarcastic in their politeness? Are they telling us, "We think you should do as we ask no matter what and feel no need for pleasantries. However, just to be funny we will ask you politely. It'll be one of those funny stories to tell at the next holiday party."?
I think from now on when I ask someone to do something for me I will insert air-quotes around the please just to see the person's reaction.

ps. It is January 28 and it is raining.