Just a paying customer, albeit one with quite a lot of Customer Service background, and I'm frankly astonished at some of the attempted explanations / justifications for UAs actions here.
They messed up, then messed up some more. Then their CEO opened his mouth and made it worse. The he opened it again and made it much, much worse. He's slowly moving this from an embarrassment, to a crisis, to something that puts his job at risk to something that could put the whole airline at risk.
This should be really simple. You're a commercial organisation. Your customers come first. Especially when they're already sitting on the aircraft. If you discover that you have to move some of your own staff around, that's not your customers' problem, it's yours. Sure, offer a bribe to see if it will solve your problem, but if they won't bite, you have to sort it out yourself.
Your need to get your staff somewhere does not outweigh your customers' needs to get somewhere.
If you, as a provider of a customer service, think that you have the right to mandatorily - or even forcibly - remove a customer from a plane in these circumstances, you need to take a good hard look at yourself. Because you don't.
Oh, your CofC may say that you do, but who cares. We all know that CofCs are there for the sole purpose of limiting your liability in the event of a problem. No-one reads them because there's no point. What's going to happen if I decline the Terms? You won't sell me a seat. I need to travel, so I have no option but to accept.
Same with nuances about legalities of the point of "boarding" or rights of the Commander. Very interesting to the lawyers I suppose, but of no interest to the public.
What we see is a man being physically dragged from his seat because UA's desire to move some staff around was more important to them than the people who'd paid good money to use their "service". Then we hear the CEO complain that the poor chap became "disruptive" after being asked to "voluntarily" leave the plane.
That's what we base out judgement on. A company that's disappeared so far up its own backside that it's forgotten that it exists only because people being willing to give it their business. Treat people with such disdain and contempt you simply don't deserve to exist.