Earlier quoted context omitted.
The hard part is preventing theft, not adding numbers.
Cashiers should not, and will not prevent theft. They're not paid nearly enough to get in danger, and it is not their job. I'm sure you can find videos of thefts in San Francisco if you need a visual demonstration. No cashier is going to jump in front of someone to stop a theft.
You're doing the all or nothing fallacy. The fact that a cashier does not prevent all thefts does not mean a cashier does NOTHING for theft.