It's visually distracting, attention-grabbing shitty behaviour that undermines the actual performances on stage.
If there's an auditorium full of people around you who've paid their hard-earned money to see professionals perform, how is arsing about in your seat, miming and having a laugh, fair to them? Even if you make no noise it's annoying as hell.
And how is it respectful of the people on stage, whose entire job is to hold the audience's attention?