I don't know what it looks like in the US, but here in Europe it is often faster / more convenient to use the self-checkout. The lines are shorter, it's easier to bag everything, and it's easier to keep an eye on all the prices.

Plus there are random inspections every now and then, and usually cameras on each machine. To top it off, they don't take cash, only debit cards, so it's very easy to find any wrongdoers. I wonder if the anonymity of cash is part of the 'problem' here.