OK HAL, understand you are an Airline Pilot, and in the industry. And clearly, I am not. However, are you telling me that the authority and influence (something I believe every flying passenger understands) of the Captain is something that cannot be exercised while the cabin door is open? I think most people (myself included) would heed the "recommendation" of the Captain much more so than the gate agent or some other airline employee.
A couple of points:
1. The company policy for most air carriers specifically says these type of issues at the gate are supposed to be handled by the customer service agents and the Captain is to stay out of it unless it impacts the safety of the flight. Taking someone off a plane doesn't impact the safety of the flight. On the ground at the gate, the Captain might have influence but he lacks the authority. FAA derived authority over pax does not start until the door is closed and you are away from the gate.
2. No one, either the other pax or the company, wants delays. Delays have a ripple effect down line to other flights. If the Captain leaves the cockpit to handle pax issues instead of finishing his preflight, it will cause further delays.
3. Many times these issues happen and are handles without the cockpit even becoming aware they happened. With something as big as police boarding the plane, the cockpit would obviously know. But probably not until the police were already called.
4. If the Captain gets involved and it gets physical like it did in this situation, who is going to subsequently fly the plane. I have seen both customer service agents (on the ground) flight attendants (both on the ground and in the air) physically grabbed/assaulted by the pax, and in one incident in Pago Pago need to go to the hospital.
It really is for the best for the Captain and other pilots to stay out of the incident on the ground unless it is to demand a pax removal. Even then, we don't actually go back and confront the pax ourselves.