Yeah but you're forgetting step 0.5:
0.5 Hey, Farmers, Uncle Sam here... we've discovered that we can radically influence the domestic and foreign affairs of half the world by using food as a carrot and/or stick. Therefore, the more food we have, the bigger the carrot and/or stick.
The rest of it flows from that. Our agricultural policy is designed around massive surpluses for tactical export. Whenever we have too much at home it's because everyone else is doing fine without us, much to our regret. Whenever we have too little at home...
wait, what am I saying. We never have too little at home.