I'm told that theaters make their money on food. As long as people are there eating popcorn and drinking soda they profit so it doesn't matter who those people are or what they are seeing. It's not as if theaters pay more for heat, water, and air conditioning if it's the same asses in seats vs new ones.

On other hand you could theoretically fit double the number of people if not more in 1 hour film vs 3 hour film.. And they likely would buy almost same amount of drinks and popcorn.