Because some parents are idiots who don't care if their kids have nightmares or not. Or that other people have paid nearly 10 euros to watch a movie.
I went to see one of the remastered StarWars movies a good few years ago and one little brat down the front was jumping up and down in the seat waving a lightsabre around. Well, at least until one of the guys behind me stood up and shouted down that if the people with him didn't stop him they would have the lightsabre stuck somewhere painful. He got a round of applause and the mother dragged the kid out of the cinema in a huff.