We started noticing this happening at the local mall food court this year: a group of people, having finished eating, just get up and leave, not bothering to toss their trash or put away their trays.
Hey dumbasses! I know at some places you can leave your crap at the table, namely buffets and many proper restaurants like Red Lobster, but a food court is not one of them!
Here's a tip on how you know you're supposed to toss your own trash: look for trash bins. If you see trash bins around the seating area, guess what, you toss your own trash! Don't be slobs and just leave it there like a bunch of pigs!
I know we could have told the people this (in not so harsh words), but these days, doing so could get you ignored, flipped off, or worse :/.