I think there's some serious questions regarding the police officers' response. After briefly checking California firearm laws, it does not appear that owning an assault weapon is illegal. Open carry is partially allowed, but that seems to apply more to rural areas. Even so, does the act of illegal open carry of what is potentially a legally owned firearm justify assuming crouched positions behind open car doors with guns drawn? Would they have behaved similarly if the weapon in question was a handgun? This situation seems to have been escalated before the officers even knew what was really going on.
Second, how relevant is whether or not they told him to drop the gun? This is California and the boy would seem to be of Latino descent. Is it difficult to believe that English might not be his first language, or even one he knows at all? I'm reminded of the deaf man shot by a police officer when he didn't notice calls to drop his whittling knife. Barking out orders at people unaware of your presence won't always illicit immediate response even if they do share a common tongue.
More generally, I'm not sure there should really be many scenarios in which a police officer is the first to shoot. There are far too many unknowns for me to be comfortable with officers who shoot first and ask questions later. It shouldn't be difficult to run through some quick mental probability calculations. How likely is it that someone walking down the street openly carrying an object shaped like an assault rifle is intent on committing a crime? Is it more likely that they might simply be transporting it? Given its size, it seems openly transporting a rifle would be more likely than openly transporting a pistol, for instance. What's the chance that it's not a gun at all? How often do you see 13-year-olds handling rifles versus toy guns? This shouldn't be a situation in which police immediately assume hostile intent and respond in kind.