Your assumptions on typical behavior do not match my experiences. I normally bring my own bags, and because of that, I pack the bags while the clerk scans the

Your assumptions on typical behavior do not match my experiences. I normally bring my own bags, and because of that, I pack the bags while the clerk scans the items (as opposed to having that person do both tasks). This greatly increases the speed to checkout. And more often than not I see the same behavior from others who also brought their own bags (and at least in SF, bringing your own is very common). Also, I worked in retail when I was younger. Most bigger stores keep an eye on the lines and will move more employees to the checkout registers if the lines are getting too long.