The main reason we have food stamps is to offset the cost to consumers of agricultural price supports, such as the high prices of milk, juice, bread, etc. that everyone is here complaining about.
Both food stamps and the commodity purchase program (that Brutus talks about) are primarily farmer subsidy programs. That they also provide welfare to the poor is incidental.
The commodity purchase program, by the way, provides really good food to those who can get it. Invariably, the reason why people don’t get commodity is because their community has not bothered to set up a distribution network. The law makes this responsibility a local one (feds pass to states, states usually to local government), and generally speaking the local government has fallen down on the job. If you think more should be done with commodity food, complain to your mayor/alderman/county commissioner/township trustee/other local government official. It’s their responsibility to be distributing all this free food that is rotting in warehouses. Maybe you should volunteer at a food bank or soup kitchen and then try to get them involved in that distribution. Food banks and soup kitchens are generally eligible to receive commodity food, but it can be hard to pry it away from the people who are holding on to it for no good reason.
Also, let’s not forget that the thousands of military families who receive food stamps. Are you going to tell the families of those who serve our nation that they’re only allowed to have PeopleChow<tm>?
A final point about soda: the water in many communities is unpalatable, or at least unpleasant to drink, and many food stamp recipients live in substandard housing with inadequate plumbing. You cannot buy bottled water of any sort on food stamps. (You may be able to get it on WIC, if you do not have suitable water, but generally this is difficult to arrange and requires that you know your rights as it is unlikely your caseworker will explain it to you.) Soda (generic store brand, at least) is generally the least expensive by volume beverage sold in the store. Buying it makes sense to me.