What is the etiquette with using headphones on flights?

The last 4 flights I have been on people around me have been (or their children have been) watching a film or TV show out loud, no headphones for the duration.

To me this is the height of selfishness and I would never dream of doing it. If I forgot my headphones I just wouldn't watch a film, but seemingly everyone else thinks it's normal and fine - I haven't ever heard anyone complain to the person and the staff don't say anything.

Is this totally normal?