Well yeah, you're right that foodstamps do not pay for themselves. But it's only 3% of our budget, and it provides the most possible economic stimulus, so it seems to be the best value for the money.
We the people, our US government, elected by the people, decided that setting aside 3% of our budget so poor people didn't have to go hungry was a good idea. And it works. And business like it, farmers like it, retailers like it -- it benefits alot of people without costing alot of money.
If we eliminated the entire food stamp program, it wouldn't even cause a dent in our national debt, but it would negatively hurt the economy.