Why would anyone clap in a movie theater? If you’re at the Cannes Film Festival or Sundance or at the red carpet premiere of the film, where the director and the actors and the screenwriter might be present, it would be more than understandable to clap at the end of the film because you would be voicing your approval directly to the artists.
Maybe they are applauding the projectionist, for making sure the transition from first reel to second reel went off without a hitch? Maybe they are applauding the Cineplex, for their clean theaters and their comfortable seating?
I wonder if these same people clap along with the studio audience when watching TV?