Coming from outside the Airline industry, but with a strong consumer rights background, I am appalled at the treatment of this customer.
It is high time that airlines stopped considering themselves as anything other that a common-or-garden passenger transit service. Imagine if this had been a bus (which effectively it is), or a train, and a paying customer was dragged off the bus to make room for a company employee.
The practice of airlines overselling seats (albeit posters have said that it was not relevant in this instance) is absurd and places profit maximization ahead of consumer rights - it should be outlawed. In any other business you would be arrested if you sold (and took payment for) more stock than you were capable of delivering.
According to this source United Airlines has 728 active aircraft:
United Airlines Fleet | Airfleets aviation
With a fleet of that size moving staff around must be a logistical challenge, but reneging on customer contracts cannot be the answer. A consumer who has bought a ticket, and is not a threat to anyone else, has a right to travel on the flight that s/he has booked and paid for. End of.