That’s a US thing. In Germany it’s common practice everywhere that the cashier does the scanning, you do the putting in your cart, or wherever, but if “wherever” is slower than a cart you’ll get death stares from other customers. They probably introduced that to deal with Americans who’d otherwise just stand there twiddling their thumbs.
Also ALDI cashiers have gotten slow: They introduced scanners very late, before that cashiers would rummage through the belt with one hand and enter four-digit codes with another. It was possible to keep up with fresh cashiers but the seasoned ones were absolute speed-hogs – not that they’d mind you being slower, they just were done quickly because with practice, you get blazing fast at code entering.