There's a Lesson in Here Somewhere

Elder feral son Uday ate all the chocolate chip cookies the neighbors gave me as a thank you for checking on their house while they were away, so I bought some more and put a sign on the package with one of his childhood pictures behind an international prohibition symbol. I found him chowing down and insolently grinning at me:

This tells us something about signs that say "No" vs. people that don't give a damn.  But what?