It happened in California... not shocking really.
I understand many will find my attitude on this subject somewhat... dry, but frankly if the gun is the same one displayed in the pictures, given the laws in the state of California, the police were within their right to fire feeling threatened by a kid with an AK. That picture clearly shows that the "Orange/red" tip that should be on the weapon is not present. The kid was probably being a smartass and waving it towards the cops too.
Is it terrible a 13 year old kid is dead because of this? Yes, yes it is. But it seems, as it was, that the kid [or his friend, as the gun apparently belonged to his friend] was already breaking California state law by not having the blaze orange tip present.
Frankly we don't have this worry here in the state of Arizona. You can open carry an airsoft gun just as simply as you can a real steel version.