If they work N hours and receive fewer than $7.25 * N, then the employer is required to make up the difference. I understand there are some accounting tricks involving another $2.13/hour floor and tips, but regardless of those tricks they can't be paid less than minimum wage without the restaurant violating labor law.

These positions were in Western Washington outside of Seattle. Standard minimum wages apply and Amazon has a history of paying above min wage.

No, the main point is to get labour at a lower total cost, even though you can't predict very well how much you'll need.

Sign up to a call center, and they'll say "we'll answer your calls and bill you per call. If you want to do it yourself you can, but in order to cover the peaks you'll have to hire so many people that most are idle during the troughs... we know how to plan this and scale and things and more things, and you don't so why don't you just let us do it?"

Getting around irritating minimum wages and other laws is a fringe benefit. Scaling the headcount is the big one.