Capitalism is the reason there is any food to waste. With communism, your lucky to get scraps.

The reason they throw the old food away is because it has gone bad. Food that has gone bad is dangerous to eat. Its basically poisoning the poor, and the people who give them the free food would be liable for any injury that they received from the expired food. There is some exception with the "Best buy" date (the food starts tasting worse but is still safe to eat), but effort has to be made to insure that the food won't harm the people eating it. There are some laws in place to protect supermarkets for this (Good Seminarian food donation laws), but it is pretty common to not want to risk lawsuits with fairly minimal gain. Moreover, American food waste from retail is less than in France (which lacks the protection and financial intensives that America has for donating food) and already has enough donated food that it should theoretically cover all food-insecure people in the country safely.