I’ve seen it happen. Mostly with pennies, which I kinda understand because they’re so damn worthless (an entire roll of 50 pennies won’t even get you a cheap candy bar these days).
Still, I only remember seeing it happen after people have left the store, not right in front of the clerk.
I always stopped and picked up the change, BTW 
Taken literally, of course, this would mean that you can’t drive through and pick up dinner for your family without loading them all into the car with you. I’d like to see them enforce that! :dubious:
I’ve also never heard of anyone deliberately dropping change. Like Shodan, I do usually put pennies into the slot for Ronald McDonald House if I’m driving through McDonald’s–which I do, far too often for my own good, at breakfast time.
Never heard of it, even as an urban myth.
However, I suppose I could see leaving it there if I did drop it on accident. Not all drive-thrus have space to open my door. Even if I could get out of the car, there’s a good chance the change rolled underneath. I’m not getting my clothes all dirty scrounging for change… and if I drove forward, one of the cars behind me would just take my place. Plus, the longer I take getting my change, the more I’m inconveniencing other people behind me.
You must not be familiar with the OP.