I guess it depends on how you set them up. In the UK you'll find self-checkout scanners in nearly every supermarket that contain scales. Stealing with those isn't easier than just stealing in the first way. You can't just "forget" to scan an item.

Scanning while you shop, however, doesn't really seem to work. Tesco tried that and basically stopped it again. You either need to check really often (high staff costs) or will have losses.