Well, that does take a lot of nerve, but the world is full of nasty people as well as nice. I would imagine that the vast majority of the population would never dream of doing something like that -- unfortunately, it is the few bad apples that we remember. I worked retail for over 20 years, and I had thousands of very nice customers -- having been out of it for about 5 years now, I can truthfully say the handfull of really nasty ones, the ones that made me cringe every time I saw one of them walk through the door, are the ones I remember the most.
I guess the only real thought I have about trying to prevent this is 1) make sure your cash box for accepting the money is somehow secured so that someone can't walk off with it and the cash and 2) perhaps you could install some kind of a surveillance camera, or even just a dummy one (non-operational, but with a light and motion so it looks like it really works, available for like $15 at hardware and discount stores), and a sign that says "Video surveillance system in use. Drive-Offs will be prosecuted" (like at the gas station). Unfortunately, no one really will ever be proscecuted for a $3 theft, because no cop or DA would want to bother with it, but maybe at least that would deter them the next time.