The fact of the matter is, young children are impressionable. It doesn't matter the content or the media, if they see something that strikes their interest, their actions will, to one degree or another, incorporate it. That's why you hear little kids toss out a curse word if their parents curse a lot. It's why kids run around playing Power Rangers in the playground. Yes, most of these can and are innocent and short lived, but like with every thing, there are the outliers - the emotionally unstable person who misses the mark in terms of what is right and wrong in their actions, influenced by a life of unrestricted media and inappropriate action.
"Yes, 9.999 out of 10 kids aren't going to pick up a gun and start shooting people because they played Call of Duty, so we don't need any research or regulations!" gamers will say (and being a gamer, I would agree to an extent) but doesn't that sound an awful lot like what your news-spread pro-gun people say?
I guess my point is, anyone who blindly says "What I love isn't harmful! Look at the other guy!" is fooling themselves.