I think the issue is two-fold. First, many overweight people drink a lot of their calories, so a prohibition against carbonated beverages should cut their intake of those. Secondly, and this is more in line with your desire to drink the diet stuff, is that for some reason, carbonation doesn't feel good in a sleeve or a pouch. There's no reason I can't drink a soda, but I become a burp machine if I do and the feeling in my sleeve is usually uncomfortable. That said, once or twice a year I have a root beer. Mostly what I drink now is Crystal Light.