Yup, because food becomes so valuable.
You forgot to add that they also ration that food.
Now imagine how much gold, TV's, cars, worthless money that stuff can bring you on black market!
Just ask anyone who was in peacekeeping or humanitarian mission:
A couple of food packs, few litres of petrol or box of antibiotics can buy you pretty much anything you desire.
You can't buy much for golden Rolex or $ tho, it's useless to people!
