Forcibly? Last time I checked no UA worker laid a hand on the man. Also, they weren't standby's, the were positive space confirmed, as in they had a reservation.

Believe it or not, LEO's are not called as a muscle squad. They are called for the safety of the workers and surrounding passengers, as well as someone who can talk with the passenger as an officer of the law and convince him to comply with orders.

Once again, something you would not know since you haven't been a gate agent and dealt with irate passengers.