A thief doesn't know what a cashier will do. And a cashier is an eye witness or can yell "hey stop them!"

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.

> The fact that a cashier does not prevent all thefts does not mean a cashier does NOTHING for theft.

Yes, for one thing, it ignores that a very large share of retail theft is insider theft, and that cash handling positions are the largest portion of that.

Cashiers absolutely do something for theft.