Kopikatsu said:
...for many reasons!
It would take forever to go over all of them, so...I'll just give an example of what happened at work the other day. There was a guy who ordered a large soda, so, I gave it to him. Then after drinking part of it, he asks how much it is, despite there being a GIANT menu sign behind the counter, and so I tell him, and he says "Nah, that's too much."
So...he dug through the tip jar for the cash, handed it to me, then left. I was so stunned that I didn't even say anything...and he didn't even give me enough to pay for the thing. (Sales tax...)
Why are people such assholes?
Well, I have more faith in humanity than you do, but to address this situation this is probably your fault. In general when working at fast food places, it's common sense to not hand over the food until you've been paid, specifically for this reason. A petty thing, but if I was your manager I probably would have chewed you out for that. Indoors it's less of a problem, but especially at drive thrus there is a problem with people handing over the food first and having the guys drive away before they pay.
Ideally you probably should have gone outside and gotten his liscence plate number, and then called the police. Granted he only committed a misdemenor, but the police are probably going to talk to him, and get you the money.
All told what you did was a pretty common mistake, and hopefully you learned from it. I've dealt with things like that before when I worked casino security (dealing with the food courts and such), not common, but in such cases security dispatch would get a call, I'd get radioed with a description, and me and a few other blazers would go have a word with the guy. Typically followed by telling off the empolyee about being more careful, before (or after) the manager tells him the same thing.
Nothing earth shattering, just be more careful, the suckiest part of it is that the guy got away with it.