Most of the theft problems can be solved by a scale.
It is extremely easy to steal through the self check-out if, like Walmart, there is no way of weighing the items as they are placed in the bagging area.
With the scale, If something is placed in the bagging area that has not been scanned, the weight discrepancy will forstall the checkout process and alert the person supervising the area.
Without this scale and alert system, all you have to do is scan a smaller, less expensive item (such as a candy bar or mailer envelope) with a more expensive item, letting the machine scan the bar code of the less expensive item. If anyone is watching, they see that you scanned it and the machine registered something. Or, you can bypass the scanner entirely.
But if the weight is a factor, and the scale knows what was scanned is supposed to weigh, it is impossible to place an unscanned item into the bagging area without alerting the supervisor.