Why Isn't Violence Considered Pornographic?

It's interesting that the newest purity pledge being required of Republican presidential candidates includes an anti-pornography element. By "pornography," what they intend is surely images of nudity or sexual activity designed to arouse. But why do we segregate off these images from violent images designed to arouse? If you watch movies, television, even cartoons designed for children, there are weapons aplenty. People are killed or injured viciously and as long as it is "good guys" shooting "bad guys," it is deemed morally innocuous. But if we replaced one instance of violence with the image of a nipple, people go bonkers. I'm not arguing that we should be showing children nude bodies in their entertainment, but wondering why we don't have the same worries about showing them depictions of violence when a predilection towards acting in such ways does cause significantly more harm in the real world to real people than would running around without clothes (with a high SPF sunscreen because sunburns, especially on sensitive areas, can lead ultimately to melanoma).