There was absolutely nothing unlawful about the LEO reaction. You can spout off all you like about whether or not it should be legal but if you don't think a LEO can legally stop and question you for open carry you'd be naive. And when he does you'd better damn well do what he tells you too. Geez, not brain science here.
I'll go one step further, he should of been stopped, questioned, temporarily disarmed, etc. If I see him walkin down the street I may or may not call him in. It would all depend on his demeanor. If I'm and LEO, I'm gonna check him out and I WANT my LEO's to do such. Having the right to do something does not mean that we don't have a right to make sure things are on the up and up.
Apparently, you are unwilling to say that there are limits to police authority.
As I understand it, officers can only do so if there is a reasonable articulable suspicion that a crime is about to be, has been, committed or is being committed. They can walk up and start talking to anyone on the street. No response is required if there is no crime being committed (or recently committed).
In this case, the officer needed to only observe the sergeant to determine if a crime was being committed.
If the sergeant was carrying the gun safely, then the officer should have determined that he had no authority to take further action. If the sergeant was not carrying safely, then the officer would have had probable cause for an arrest.
Here in Virginia, an officer cannot walk up to someone lawfully
openly carrying a weapon and detain them. They cannot demand identification and they cannot take the handgun away from the person carrying. It is assumed in law that the person carrying may lawfully possess the weapon. The officer is free to engage the person carrying a gun in conversation, but the person has no duty to respond. Doing so is beyond the authority of the office and opens up the officer, department, and jurisdiction to civil liability at the minimum.
This is well settled law in Virginia and we take it very seriously. We go after any officer that violates these rules. It's easy to prevent too; we just do some officer education on the side. Unfortunately, this usually happens after an incident, but organizations like VCDL have done outreach in the past.