Exactly. If you look at the fine print in United's "terms of carriage" the process for dealing with overbooking BEFORE BOARDING is fully covered, as are the terms for denying carriage AFTER boarding... which do NOT include overbooking as a valid reason.
By the time boarding began, United should have already known how many crew would joining the flight and should have dealt with an overbooking situation BEFORE the boarding.
This was a screw-up by United in that their procedures for employees needing to deadhead somewhere should have to arrange their trip with their employer in a timely manner... before scheduled boarding time. If an employee shows up at the gate after the plane has pulled back and is waiting to take off should they bring the plane back, kick someone off and delay the flight for hours to accommodate the employee who for whatever reason didn't schedule their intent with the airline in advance?
There needs to be balance between the needs of the airline and its employees and the customers... I think United crossed that line this time.
If an employee shows up at the gate after the plane has pulled back and is waiting to take off should they bring the plane back, kick someone off and delay the flight for hours to accommodate the employee who for whatever reason didn’t schedule their intent with the airline in advance?