I think the theft they are trying to catch doesn't come from the employees as much as the non-robotic cashiers.
And you are also correct that there is a great deal of theft going on in the robotic check lanes.
I mean a small example is that I could type in 4011 for any banana, but still have taken the more expensive organics for example.
That's a small variance in cost, except when you consider that 4011 is the number one seller for the implied greedy, stingy government welfare recipient company.
All the same, it's the property of the customer and by stopping to see a receipt cop, people are actually waiving their fourth amendment rights that protect them from unwanted search or seizure.
I know well that no smart shoplifter is going to put stolen property in the bags as much as they would conceal it. Besides, a shoplifting case is better made from camera reel or from the eyes of a security guard than a greeter turned receipt cop.
It's okay to keep employees honest....it's just the realization that since it's my property after I paid for it, that compliance is voluntary and not compulsory... and a shopper/citizen does not have to waive their right if they choose not to.