But that's exactly the behaviour we want to encourage, when we consider so many poor people in America have a huge calorie surplus (ie. are obese or overweight)

But that's exactly the behaviour we want to encourage, when we consider so many poor people in America have a huge calorie surplus (ie. are obese or overweight). If 1kg of Oreos (41% sugar) costs $3.60 instead of $3.19, people will spend the same but just consume 11% less, or manufacturers will recreate the recipe to have less sugar. Both of those outcomes are good. The money collected can be used to subsidise community kitchens, or raw fruit and vegetables.