I agree that him getting back on the plane was bizarre, but that isn't the issue. The issue was that the airline decided that the only way to solve this issue was to forcibly remove a paying customer from their plane, after doing nothing wrong. You think he should've been tased or maced? C'mon.
How else could this have been diffused?
Hmmm... maybe they could've kept on upping the offer for anyone to leave the plane until someone actually accepts the offer?
Let's just imagine for a moment that this 69-year-old man, was your 69-year-old grandmother.
Let's say that United asked her to leave the plane, and she's no, because of one reason or another.
Would you honestly think it would be ok for a cop of grab her and drag her down the aisle, off of the plane??