the issue isn't with the agency one has over the expression, the issue is the expression. you're bothered by expressions made after the movie is over - an expression which in no way hinders your enjoyment of the movie - but not ones made during the movie - which could lead to you missing information.
maybe you don't want to criticize an action you take part in, I don't know. you asked why people do it - it's an outward expression of satisfaction with the movie.
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ICON:
When there's nothing left to burn, you have to set Bidoof on fire.