Suppose(I do not know) he would be legal walking into Wal-Mart.He IS going to get LEO attention,legal or not.Legal or not,it is stupid to walk into WalMart displaying an AR.
He wasn't in Walmart. He was on, assumedly, a country road of some kind.
Let's go the other way- Had he been in the middle his hunting lease during hunting season, and an anti-hunting neighbor called in- then what?
We don't have enough information about what happened to make any judgement calls either way. Though I haven't seen anything where the LEO asked the guy to set it down and he didn't- Just that they walked up with guns drawn and took it from him.
The LEO can say anything he wants until video evidence proves otherwise. And he's likely to with that problem of holding the child for questioning without parental approval.
Your argument appears to be "It Takes Two To Tango". And the flip side of that is that the LEO could have handled it a whole lot better as well.