I'm an adult and have no children, but even I don't want to watch graphic violence in a space where I can't get away. I've been in similar situations, and I can mostly distract myself with a book or music, but I still catch some of the movie even without trying. Even the best behaved child will do the same, and other children will actively try to watch. For those saying "just distract your child", do you know how difficult it is to distract a child who is determined to do something? I don't believe in changing every little thing to make it acceptable to every parent, but I do draw a line at graphic violence and sex.

Also, not showing graphic movies does not equal only showing cartoons. There are plenty of other adult movies that are not visually graphic and would be much more universally accepted. As others have mentioned, it's a million times easier to withhold headphones than to try to stop a child from watching something in their line of sight.

Now, it's possible United edited out the most graphic scenes. In that case, I'd be willing to give more leeway on what's shown. I just don't know without seeing it for myself.

Finally, PG-13 means that some material may be inappropriate for children under 13, and parents are strongly cautioned. (PG means parental guidance suggested for younger children.) In my interpretation, PG-13 does not mean just explain to your child why what they're seeing is wrong; it means it's inappropriate for your child to see in the first place.