What if employment law advances such to add liabilities to human employment? Then you might get payed $3/hour, but you costs include insurance, losses due to sick-days etc.
I guess, but you must also consider the upside as well. A human cashier is not just a cashier, but an ad-hoc customer service rep, security guard, market intel gatherer, etc.
I guess this is the "human touch" argument, whether this actually applies varies; but for "market intel" machines are way better at gathering information.