If people want to waste their limited government food provisions on Soda, then by all means let them waste it on Soda.
That's a great sentiment until some government bean counting group determines that the current level of funding for food stamps isn't sufficient to ensure that recipients are receiving their RDA of various food groups, or whatever criteria they decide to use this year and decide that they must increase funding.
Most people who receive food stamps do have jobs. They just earn below a minimum that the state decides qualifies them for food assistance. They are free to choose to spend their own money on soda if it's really that important to them to have it. I don't think it's unreasonable for a program with the stated intention to prevent people from going hungry due to lack of money to buy food to limit itself to only those things necessary to prevent people from going hungry due to lack of money to buy food.
I just find this whole discussion interesting because my family grew up pretty poor. We never were on food stamps, but guess what? We never drank soda either. We drank water. Sometimes, we mixed in some instant tea for a bit of flavor. But mostly we drank water. Do you know why? Because we could not afford it. The very concept of people on food stamps complaining because they can't buy soda with the stamps is laughable to me. Do without. You wont starve to death if you don't have soda to drink.