Skinny people dont drink diet sodas because they dont have to worry about their weight - if you are skinny and not prone to weight gain you are not going to get fat by drinking sugary drinks or sugary stuff if you eat/drink in moderation
Fat people drink diet soda because they have a bad diet full of sugar, since sugar is fattening they drink the diet version in order to reduce sugar intake to lose weight
So Joan Collins should think about cause and effect a bit