I remember watching World War Z (2013) whilst aboard on a flight, the latter half of this scene was removed just because it involves Brad Pitt’s character detonating a grenade inside a plane because of zombies. I mean why is it that movies before being shown on planes have to cut out specific scenes despite it being a work of fiction?
You got some good answers already, but I feel like what’s missing is “so they can say they did something.” That is, if they do nothing, people complain (like the comment about the Karen who saw nudity on their neighbor’s screen), they have some recourse. “We edit what we can but at the end of the day, you can’t call the police because you looked in your neighbor’s window and saw them watching a porno or a violent movie, it’s no different on the plane, watch your own screen and don’t worry about what others choose to watch.”
To clarify what someone said about countries, if you’re flying to/from the Middle East or Russia, I don’t think you can show anything gay because that’s illegal in those areas. It’s a fight they don’t want.