What have we learned from the passenger being "voluntarily removed" from the United Airlines flight?

"First come first served"? The man was already sitting in a seat. Does that mean that people showed up after he did so he was chosen (?) to be removed so that they could get on? Don't they count the number of people going thru the gate and once the number of seats are filled stop anymore from getting on and book them on a new flight or reimburse them? I also heard (on the news ?) that prior to the incident he was loud and disruptive and that people complained and when they tried to remove him he became belligerent and abusive. And that the airlines used the excuse of overbooking to remove him. Why the hell would they need an excuse if he was ?