Because their boss ordered them to do that?
There's a large discount department store in Hilo, Hawaii, that has banned people from entering with an oversized bag. They have a sign about their policy by the entrance.
If customers wants to shop at that store, they need to place their oversized bag in a locker that's free of charge. The problem is those lockers are too small for most backpacks. I've seen one of their customers struggle to retrieve their backpack.
The other problem is not all foreigners can understand English well, and I've seen the receipt checkers by the door yell at those foreigners for not reading their stores policy.
"Read the sign! A rude receipt checker yelled at a confused foreigner.
"You can't enter this store with a backpack!" Another rude receipt checker yelled.
Coincidentally, I've noticed that stores main competitor has been busier than normal with paying customers...