Whether this was deliberate overbooking or they had a situation where they needed to get a flight crew someplace else doesn’t matter, what matters is the way they handled this. Yes, situations do come up, like bad weather, unexpected circumstances, that is true of any business or even life situation, and people and instututions face this every day. The problem with the United story is that they had a need to bump passengers (and I dont doubt this was to get a flight crew to where they needed to be, rather than 4 airline personnel flying free to where they wanted to go) but they basically forgot about the passengers in this story, people trying to get home, get somewhere, and instead of looking at it from a customer perspective, they looked at it from their own need only and said “well, we offered these #&&**$ people 800 bucks, wouldn’t take it, got no choice but to ‘volunteer’ them to get off”.
Did they explain why they needed volunteers? Did they think of offering let’s say 800 bucks, a hotel room for the night and a ticket on the first plane out in the morning? Or offer more money? No, they made their offer, no one wanted it, and at that point 4 passengers became ‘excess baggage’ so to speak in the face of their need. If they absolutely needed those seats, why didn’t they make the passengers a sweetheart deal when the initial offer wasn’t taken? Probably because even though they were the ones inconveniencing the passengers, they figured they had “dieu et mon droite” to tell them to leave, probably to save the airline money no doubt.
And the reason this is resounding with people is pretty obvious, despite all their slick PR campaigns with smiling personnel and so forth (surprised they haven’t come back with "fly the friendly skies’), basically the airlines treat their passengers like cattle being shipped, like all we are is the dollar amount we paid to fly, and it shows every time you fly. Like I said, in this case the airline acted like the people on the airplane were inconveniencing them by not allowing them to bump them then the other way around, and their actions pretty much confirmed that.
As far as the idea that the guy resisted and got what he deserved, that is as someone else pointed out the classic response to abuse by cops the world over, that if they got beaten up, must have deserved it. On top of that airport security is not exactly the NYPD or other major city police force level of training and for all we know the airline told the security people the guy was being removed for causing problems. The guy likely has a massive lawsuit both against the airline and the security service, and I suspect you will see this one settled out of court quickly and quietly because United knows they have a major problem with this one, as much as they don’t care about the public (to quote Commodore Vanderbilt, the owner of the old New York Central Railroad “The Public be Damned”), they have to pretend they care.