If supermarkets give food away, doesn’t that destroy their business?

Not at all. There will always be a market for the convenience that a supermarket provides, the ability to choose what you want, when you want it. The nature of food surplus is extremely inconvenient – you can’t guarantee how much there will be, or choose what items you’ll get, or exactly when a product will be culled and pulled from the shelves. Most people with busy lives are unlikely to put up with this kind of inconvenience, even if the food is free, which is why food surplus will likely supply the more ‘in need’ population, who may not even be current customers of supermarket chains and therefore will not represent a ‘lost customer’ if the supermarket donates the food rather than selling it.