It's more likely they wanted to buy something that isn't able to be purchased with food stamps. Probably alcohol. Or smokes. Something to let them journey to oblivion and blot out their reality.
Nah, I've seen people like that too, and that's not the guys I ran into. Were I to stereotype, they looked more like gangbangers than strung out druggies.
They likely accepted the food stamps in exchange for...let's call it, recreational substances and this is how they turned it back into cash.