What makes you think they'll buy more healthier food instead of just getting even more junk? That's like giving more heroin to a junky in hopes that'll he'll shoot up less.
And honestly comparing people on food stamps to heroin junkies is disgusting. These people aren't addicted to potato chips. There are obviously people who abuse the SNAP program but you think people want to remain fat and unhealthy? The amount given to people on SNAP is not enough to pay for healthy foods. So given the choice between eating potato chips and not eating.. What options are you giving them?
This is called an eating disorder.
To be fair, I skip the meal until I get a chance to eat something nice, and I'll usually eat an extra big meal to make up for the missed one.