well again, they can't. the reason why food banks don't like it when you donate food is because it is grossly inefficient. they can buy food for the homeless at 10 cents on the dollar and the rest, surplus or bulk deals and the rest, sorting and processing "waste" food from say a school or regular people would take massive time and labor.
They don't even allow school employees to take the food, they actually destroy it. I know employees who have asked if they can take home some of the fruits, bottle juices, or milk, and are told flat out no. It must be destroyed. They make sure even a homeless person couldn't go dumpster diving for it. It is destroyed and made inedible before being put into the dumpster.