"Ooh, I've had that happen, too. It was 5c, but still, I'm the type of person who sums things as I go and expects a certain amount of change before I get to the register. So when I get my change, I point out that 'it looks like you forgot 5c here' and all of a sudden I'm the cheapest person on earth."
I used to work in a bank, and I'm very familiar with what you describe here. Whenever someone who shorts change responds in that way, it's a very good indicator that they're skimming and trying to shame the target into letting the matter drop. I fired more than one teller for that attitude and I'd call for the manager if I ever got a response other than "whoops, let me check that" to pointing out short change.
"Imagine if you worked at, for example, a fast food place, and managed to cheat every customer out of a quarter, or even a dime. That would add up pretty darn quickly. Especially because each individual person might not want to complain over that amount."