I'd much rather have America be considered "a fat joke" than it be a nanny state that prescribes precisely what you can and cannot eat.
Since schools are part of the government, they can set the can guidelines for what is served, since the food is paid for by the government. No one is stopping this kids from eating 4000 calories a day if they choose so, they just won't get served high calorie food at school.
I don't get how people think this is a law that outlaws certain foods, it isn't even close to that.