As someone who played airsoft for quite some time, I can say that our club was pretty militant about making sure members kept their replicas in a case unless we were at a field or at home, for exactly this reason. A cop who sees you waving what is designed to look exactly like a real gun around in public would be well within his rights to shoot you on sight.
That said, what's damning about this particular incident is the police report. The responding officers originally claimed that they pulled up, talked to the kid for five minutes, and only opened fire when he became 'combative' and pointed the gun at them. Then the surveillance video was released that showed their story was pretty much a complete fabrication. If they didn't think they did anything wrong, why lie?