Usually you can do that before you actually get in line, though. I think what they're being annoyed about are the people who get up to the cashier and then start dithering over what they want to get, not caring whether the people standing behind them in line with their change already counted out are running late or otherwise in a hurry, especially during a busy time of day.

Like the people I used to see in Seattle sometimes who got in the six-items-or-less express line at the grocery store with a full cart, and all I had was some deli turkey, some sandwich rolls, and a gallon of chocolate milk.