Well, in many states a restaurant doesn't have to pay minimum wage as long as tips make up the difference. And it certainly is legal to charge restaurant employees for food (it's common to give a discount, and/or offer some items for free).

Granted, McDonald's employees aren't tipped, and unless somebody eats an incredible amount, their food bill won't cut into their paycheck all that much, so McDonald's isn't able to really lower its labor cost all that much. But you could see something similar to the hypothetical at a much classier restaurant.