Everybody has a negative reaction to the story of a passenger on a United flight who was asked to deplane to make room for United employees who were needed at the flight's destination in order that a flight originating there could take off. I'm no exception, at least in my initial reaction to the person being manhandled and dragged off.
On the other hand, what were they to do if they requested the passenger to deplane, were refused, asked again, were refused again, and then had a security officer arrest the person for non-compliance, whereupon he still refused to budge? How else to get aircrew to the next flight?
Generally, police won't take no for an answer, they cannot afford to do so. At some point people who try to bluff sworn officers discover this, usually to their extreme discomfort. The officer behaved as trained.
If United didn't like the negative PR they shouldn't have called an officer. Offering a larger bribe would have cost less in lost good will and likely produced a volunteer.
I've decided if I find myself in this situation my counteroffer to the airline is as follows: you may have my seat if you'll refund the price of my ticket and fly me to my destination on the next available flight gratis. If the layover is more than a couple of hours I want meal tokens and maybe a motel room if it is 8 hours or more. I suspect United will have learned (the hard way) to be very agreeable.
Remember the old adage - it is easier to ask forgiveness than to get permission. If an airline doesn't issue a boarding pass, they don't have this dilemma. It is easier to apologize for a screwed-up reservation than to evict someone off a plane. Good planning would have kept the four bumped passengers from ever boarding.