I do not agree that the film is only about gun violence, gun culture and militarism. It also spends a great deal of time examining the influence of the media upon American citizens, and the manipulative and savvy way that media keeps Americans in a perpetual state of fear...so much fear that they helplessly cling to anything that makes them feel empowered to fight against the unnameable enemy (which is variously portrayed as disease, assault, rape, murder, fire, natural disaster, and the oh-so-scary presence of African-American males in the neighborhood).
Marilyn Manson is interviewed in this film and is very articulate and insightful about these issues. He states that American culture is based upon fear and consumption: that these two impulses work together to keep people enslaved, and that the media promulgates both, and I think he is absolutely right.