When I worked at a small cinema we would set up the movie to run regardless, because sometimes you would get late-showers buying tickets at the front desk and it's much more trouble to have to speed-start a movie for the projectionist than to be able to do it at the regular schedule. If you start it too late without manually remembering to forward past ads and trailers, you can also risk spilling into the next timeslot causing a pileup of delays. It's far simpler to just start the movie for an empty hall, and let customers join after it's started if they want to.

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.

Studio contracts: The movies are delivered digitally on encrypted hard disks and when playing there is a ton of telemetry sent back to the studios. They are watching the theaters like its 1984. Studios have contracts indicating the play will play X times no more and probably no less(else studios might hold back the good movies). AMC keeps it simple. Play the movie even if no one shows up. AMC in particular uses laser projectors now so who cares. They ain't burning out any projector bulb.