These employees are hired and compensated under the conditions of the contract between the employees and employer. They are not hired 'to directly add value to Apple during every minute of their assigned shift'. If Apple isn't getting the desired value out of their labor pool, that's Apple's problem - the employees owe them nothing.
EDIT: How many more people are going to say 'it's their choice to bring a bag, they know the policy'? The mere fact of having a policy doesn't mean it's reasonable, just, or legal.
Nor does spouting your opinion of it being illegal. As many have posted, this is common practice and well established as legal.