The use of weight, though, prevents accidents (items in cart that you didn't scan properly etc.) and also ordinary unplanned shoplifting. A careful shoplifter can weigh items in advance to fool the scale system, but for them the visual checks and random checks should make this a bad idea. Having unscanned expensive items and not having supposed scanned items which happen to weigh exactly the same should be enough to charge you with shoplifting (and of course they have the ever present video, sadly.)