I actually had no idea this was unacceptable, even with being a cashier for several years.   Anyone who did this intended to pay and made sure whatever they were eating wasn't a gross greasy item that the cashier wouldn't wanna touch.  The only worst offenders I ever had were Always moms with children.   Most of the time it was the death fat low life welfare mombies with their hell spawns, so sadly the behavior,  while not condoned, was expected; but every now and then some human mom and her child would display shit behavior with this and the mom had a case of mombie syndrome, where she instantly assumed any complete stranger with a vagina would fawn over her child, even if it was behaving badly...  I too leaned this isn't the most socially acceptable to do, but I still do it time to time as a shit lady.   My reason being from personal experience,  because honestly i didn't care as a cashier, as long as people were respectful and honest...  Aka had full intention and means to pay AND didn't make a gigantic mess.