The "Dollar General" did get hammered for it here by several states, but apparently only because their staff refused to make a refund in those instances when they got caught, much like the un-named fast-food franchise I showed receipts for.

The market here does make good when they are caught so that is at least a step over the legality line.

The issue is you have to catch them, otherwise they slip it by in the quantity of items on the receipt for, say, the weekly shop. It's free money to them, for who checks each item on a grocery receipt?