The only part you have right in the statement is that they were four employees.
The rest of the story is that they were four Airline employees who were required to be in Louisville, KY and on a rest schedule prior to their having flight responsibilities themselves the following morning.
Now, that in no way justifies what UAL did in this case, I'm just correcting the factual record here.
In Tuesdays New York Times story on the fracas, Uniteds CEO acknowledged, this was not technically an overbooked flight. And the reason for that equivocation is also clear. The four airline employees who needed the seats, presented themselves to the gate agent after the flight was boarded. Further, they were not fare-paying passengers, therefore not booked.
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In this case equivocation=lie.