For the record, and to clarify something that virtually nobody has pointed out: this was a United Express flight, not a United Airlines flight, operated by a contractor company called Republic Airways (no relation to the original Republic Airlines, which no longer exists). That doesn’t mitigate what happened, and the flight was operated on United’s behalf, using its livery and branding, which makes United at least equally responsible.
But, it’s a factual aspect of the story that has gone unmentioned.
I don’t know which employees — Republic crewmembers or mainline United customer service employees in the terminal — were the ones who made the call to have the passenger taken off.
Apparently, the passenger was removed to accommodate a United or United Express crewmember. To be clear, no carrier would deny boarding to a revenue passenger in order to accommodate an employee riding on his or her leisure time.
(Really this is problem across all of commercial aviation, not just within the airlines.