But then couldn’t you just as easily say rather than ‘people use diet products because they’re overweight’, that ‘people are overweight because they use diet products’ ? I’ve certainly heard both propositions before. “Never seen a skinny person drinking Diet Coke”

I would disagree. We should attempt to validate all such claims. Personally, I’m pretty skinny. In fact, I’d like to gain some weight. I’m also reasonably health conscious. I don’t drink all that much soda. When I do though, it’s Coke Zero (or equivalent), not regular coke. I recognize that we have too much sugar in our diet in modern life anyway, so I cut it out where I can. However, health consciousness likely leads to people consuming fewer sodas in general (and more water), so the percentage of purchased product will likely skew towards people with a soda addiction/fixation who are searching for an alternative that doesn’t require them to change much in their lifestyle.

Life has too many variables to make assumptions like that. We should seek to varify claims instead of assuming our first thought must be true.