Why do people argue with us about this stuff?

We have a couple of very particular regulars who've been coming in to our restaurant several times a week for about ten years. I remember dealing with them the first time I worked here, and the wife especially has only gotten more particular and difficult in the intervening years. Her husband is usually fairly mellow, but she gets wound up over the smallest stuff. This is a woman who tells servers "shouldn't you be cleaning or something?" if she sees them standing still; who requests the table next to the staff meal table so she can eavesdrop and tell the manager things she overhears; and who insists on writing a check every time they come in, even though we stopped taking checks about seven years ago.

I didn't have to wait on them Friday night; I was only in the vicinity of the big issue. She wanted a hot fudge sundae, and wasn't happy with the way it was served to her. She proceeded to complain to her server and describe the sort of glass she wanted it in; when her server said we don't how those tulip-shaped sundae glasses, the woman went to the bartender looking for support. Even after the bartender told her, her server told her, and I told her that we don't have those glasses and we never have, she kept insisting that the last time she was there, that's how she got her sundae! I guess she had a magic server that day.