That is a banal comparison. When a film makes you sad, you are aware of what is going on. If you are unusually sensitive to these types of emotions, you can read about the film ahead of time to see if you might want to avoid it.
Because films’ promotions may deliberately conceal information about tragic events in the story to achieve maximum impact and nobody thinks that is unethical.
It's hard to get a lot of positive reinforcement by interacting with like-minded others at scale through a movie. Facebook's original stated intent was to study contagion of emotion, which seems to me to suggest a multiplayer, interactive effect.