I'm not sure if the money my supermarket saves by getting rid of humans (and the humans' incomes) translates to cheaper groceries for me. I'd believe it if there were somehow a fine for using a human checkout. But there isn't. So every once in a while four bell pepper somehow scan as three. Accidents happen, full plausible deniability.
Because there are easier, less abrasive ways of disincentivizing this behavior, namely by making fewer human checkouts available.