When you're in theater, do you often have people who disturb the movie?
Once, there was a group, on the first row, who started to scream and laugh for nothing at each scenes. People in the back started to yell at them and all degenerated.
It made me think about a fact: Lets say your ticket cost 10$. Even if you don't know who's in the crowd, you can say that, all together, you invested 100$ to “rent” the place for 2hours, if you're with 10persons… to more than 500$ if you're more than 50persons! And that doesn't include the food. So, under that fact, it's a good idea to get security when you can't enjoy what you paid for, instead of ruining the show with all “the others”.