UAL was probably precluded from doing that by sleep rules.
IMHO it seems likely that this whole thing was caused by some overenthusiastic upper management zeal to chisel a few extra nickels off of each passenger mile by making successively more draconian and successively more impractical changes in the UAL operating procedures— in this case, to how crew gets shuttled from one location to another in advance of when and where they are needed on a flight. If the crew flight preparatory transfer is not accomplished on time, UAL is out a big chunk of money for delaying all the passengers on the commercial flight that gets delayed by not having a crew ready. (This perhaps happens often but passengers don’t get told— instead, it is blamed on equipment failure.) Someone is trying to fix a screwup. If they get the crew to the gate on time, they can bump people by offering vouchers. The crew arrived late so they were not able to bump people until after boarding (common definition). The ground crew starts to go off the rails by interpreting boarding differently than the ticket. Instead of offering more money (against another cost-saving procedure, perhaps), they arrogantly re-interpret the ticket contract term in a manner more favorable to UAL, and proceed to ask the airport authority to kick the passengers off. That is also an alleged violation of their policy (since they allegedly should have waited for police), so one can begin to get an idea of the anti-passenger mindset that the ground crew seems to have been in. ultimately it seems to have been a legal failure and training failure as well as a management failure. It seems to have been a ticking legal timebomb and Dr Dao has the misfortune to set it off.
If, at the instant that they KNEW they needed to get the four crew on board, they had notified the boarding agent to reserve room for them before passengers had boarded, then the issue would not have happened.