I was as a checker in a grocery store for a couple of years. Great job, by the way. In the mid 80s in CA, major grocery chains had strong unions so they had high pay ($18/hour at that time with time and a half on Sundays and triple time on holidays), great medical and dental benefits, and part-time employment that allowed hubby and me to mostly cover our daughter without putting her in daycare.
Most employees hated the express lane, but it was my preference. The managers were glad to make everyone happy so I mostly worked express.
When noticing someone in my line with a large order, I'd point to the sign and tell them to please use a different lane. If they had a basket full and had already started putting things on the belt, I'd make a judgment call about how long the line was, and what percentage of stuff they'd already unloaded.
In my experience, the vast majority of people with big orders in the express lane were there by accident. They hadn't noticed the sign and were embarrassed at the oversight when a fellow customer or I pointed it out. Truth is that it doesn't take much longer to scan 20 items than 10, even in those days.
Back then, the issue was less about the number of items than it was about being cash only. Checks, cash and food stamps (they were treated as cash) were the only forms of payment (we didn't take CC) and checks took dramatically longer to process. If my checkstand was idle, I'd siphon off customers from other lanes, but with the caveat that they had to be cash only so that if an express customer came up behind them the wait wouldn't be too long.
The situation described in the article shows a customer who selfishly disregarded the spirit of the express lane rule, and at least one other woman who needs some help with anger management. Good grief!
p.s. We called those sticks divider bars or order dividers.