I'm an ex-retailer. Stores are concerned about shoplifting and very concerned about internal theft. In fact, most large retailers wouldn't care much about shoplifters if losses from employees and subcontracted workers could be brought down to an immaterial level.
So, Apple's curbside pickup policy follows a procedure that is standard in retail: whenever goods or money leave the store with an employee, another employee must go along. And if the second person is not the same type of employee as the first person (to reduce the incentives for collusion, supposedly), even better. Who knows, a security guard may even be eligible for a reward if she or he stops a theft or reports an internal theft ring.