Self check out theft goes rampant.
When electronic self checkouts were first introduced, I didn't like the idea. Even today, when I use one, at least half the time, a human had to come resolve some problem with the machine. Once, a cashier had to push a button to indicate I was over age 18, because I wanted to buy a can of spray paint. Apparently people under 18 were using paint fumes to get high. The problem must have peaked and gone away, because I have a cabinet filled with about 200 cans of spray paint, but checked only once.
It turns out, there are ways to cheat the electronic check out. The machine knows only two things, barcode and weight. The most common scam is to find a cheap item that weighs the same as an expensive item. Most packages in the grocery store conveniently list the weight on the label. Scan the cheap one, put the expensive one in the bag and leave the other behind.
What's this got to do with the laws of economics, you ask?
It's one of those obvious things that are easily overlooked. It may appear that a cashier's job is to ring up sales and take the money, and that's true to an extent. It's not why cashiers are hired. A cashier's real job is to insure the customer pays the correct amount for the purchase. That is the job. The rest is just decoration.
Electronic checkouts were installed because they cost less than a human to operate, but that's only because no one thought about what a cashier's true job actually is. While they save the money on cashiers, they spend more money on shrinkage and security.