Is it rude to be rude to a rude person?

The other day, I was waiting in line at Trader Joe's in the "12 items or less" lane. As anyone knows who's been to Trader Joe's, the lines tend to be long, but they move fast, and everyone there—register attendants and customers—tends to be friendly. In fact, it's almost guaranteed that the person standing in front of me will at some point turn and ask if I'd mind if they darted back to grab something they forgot. I always say, "Sure," since I've certainly done the same thing, and it's...