This makes sense if someone bought a ticket and didn't showed up, but what if none was sold? They could just stop selling after a certain time and be sure nobody will be there late.
I'm unsure exactly how the deals with local businesses running ads before the movies are set up, but I could imagine that you're supposed to be running the ads an agreed upon number of times, regardless of ticket sales.
Sometimes in the daytime we would get retirees who would watch a movie and basically loiter around, and occasionally ask if they could catch the end of a different movie running in an empty hall. You'd sometimes let a regular crash an empty screening like this if they bought an extra snack or coffee for it or something.