They probably just like diet soda (me, don't like the syrupy throat coating feeling of the regular stuff)
The whole discussion in this subthread seems to presuppose that everyone would obviously WANT to drink regular soda. But that's not a good assumption.
I rarely drink soda, but if I'm in a situation where it's the main drink being offered, I drink diet -- mainly because I can't stand the level of sweetness in normal soda. I've had many conversations with other people (both normal weight and obese) who feel the same way. "Regular" soda just tastes terrible to a lot of people, particularly if you don't like sweet things.
Diet soda also tastes really sweet, but somehow it's not as bad. AND you know that you're not ingesting hundreds of empty calories for a drink you detest.
I think it's what you're used to. I never grew up drinking a lot of soda. But I know people who grew up drinking loads of soda or syrupy iced tea, and they just love the stuff -- they think it's refreshing to have a cold Coke or iced tea loaded with multiple tablespoons of sugar per cup.
Me -- I can't stand the stuff. But once in a while, I do like the fizz in a drink. And since one can't typically order soda with a quarter of the normal sugar (which would be about the level I'd want), diet soda's really the only commonly available option.
Or, to put it another way, what is "worth the calories" to somebody? I don't like fast food -- most of it tastes horrible to me as well. If I want a burger, I want a REAL burger (I often grind my own meat fresh at home), not a processed calorie-bomb that has to have loads of sauce on it and flavor-enhancers to convince you to eat it.
Anyhow, to me it's "worth the calories" to have a homemade burger sometimes that might have the same calories as a Big Mac or whatever. To others, they might think it's "worth the calories" to eat the Big Mac, because they like the taste, but they might not like overly sweet sugary soda enough to make it "worth the calories."
Maybe there are some people out there who order diet soda even though they hate it, just to save calories. But I also bet there are a significant number of people who just don't really like regular soda that much. Frankly, I know very few adults beyond college age who do -- maybe it's just the people I hang out with, though.