Here’s why, from Roy Spencer:
Because the man had a legal right to keep his seat.
Under United’s Contract Of Carriage (COC) rules (which follow federal rules), a passenger may only be bumped from a flight before they board (Rule 25). After they have taken their seat, Rule 21 is in effect, which would allow security to forcibly remove the passenger for many reasons — none of which includes accommodating last minute needs for a seat for other airline employees (or even overbooking).
The flyer is in a contractual relationship with the airline, and each has rights and responsibilities under that contract. United Express violated the terms of the contract, and injured the passenger in the process.
Yes, Dr. Dao has a shady history. Under the facts here, that is not relevant.