While the airlines don't always get it right I can't blame the pilot for removing this passenger if other crew felt he might be a threat or disupt the flight. No matter what when you are told to leave an aircraft you comply.

I was once wrongfully barred from a flight for a really stupid reason by an idiot pilot. This was two months after 911 flying out of Heathrow back to the states. After boarding it turns out that I had the same seat number as another passenger. We were both asked to deplane so that they could figure it out. I was out of there within five seconds. The other passenger argued but eventually complied after being threatened with arrest. In the meantime the pilot decided not to let either of us back on. I don't know what happened to the other passenger but I met a manager who thank me for my cooperation, apologized that this was a system glitch and got me rebooked and upgraded.

These things happen; last summer thirty people were removed from an Air Canada flight, some did not deserve it and were apparently removed at random. They weren't happy about it but they complied, were rebooked, put up in a hotel and were compensated and got an apology ( although in this case either the captain or the CEO should have apologized). The most important thing: these passengers complied.

On a sidebar, I find that in the US police are very quick to use force. I have have seen police remove one passenger in my life ( in Singapore). A passenger didn't want to get off, six very polite offices came on the plane, who with machine guns and with a very scary looking dog. One of the officers just said " polite first, force second", taped on his machine gun and let the dog get closer to the offender. The offender got up and left. That's how you defuse a situation by polite while showing you have the upper hand.