People Order A Large Popcorn With A Diet Coke

Better question being, why not? There is this tendency to see this as a giant contradiction. Given the question of "what does not belong?":
a) giant tub of popcorn wit extra butter
b) king size snickers bar
c) 87 oz of blue dye Mountain Dew
d) Diet Coke
Most movie goers would choose d.

Wrong answer. Given the choice of a-c, d makes the most logical sense. Why not spare yourself the extra 700 calories from a movie vat of soda?

A few years back, I had a similar run in with poor decision making when I saw a man eating a meatball sub while riding a stationary bike. The natural thought being, what an idiot- who eats while exercising. Excluding the obvious fact that it would make the average person violently ill, its probably pretty efficient execution- input and output trending towards zero.

The problem here is context (isn't it always). If I saw the same man eating the sub on a bench outside the gym, I would probably think, hmmm what a delicious sandwich. Sandwich eaters belong on benches, not on bikes. When the content of bench sitting collapses into stationary bike riding, nothing makes sense. There is a demand for identity consistency within the context. Gym people either don't eat or we imagine them grilling up 4oz of fresh salmon in their condo at home. Bench people, well, bench people sit and maybe they eat a sandwich if the sitting isn't overwhelming.

But why not allow people to be inconsistent? Why not offer a high five to the runner who stops to smoke? Maybe smoking is the only thing that makes their life as a runner even remotely tolerable. The point is that narrow contexts overly consolidate the way we think about actions and people, when really the "idiosyncrasy" of it all is what makes the most sense.