Imagine if the surgeon had informed flight crew that he was needed for important surgery, and then the captain informed the surgeon that the plane would not be flying with him aboard so the only way that he could get to the hospital on time would be to leave now, and find alternate transportation.

And further imagine that the surgeon sat tight anyway, and that a United lawyer then informed him that he, the surgeon, could be sued by every other passenger who was unable to travel to important appointments.

In other words, the guy could have been rationally talked off the plane.

In fact, United could have contacted the hospital where he works, informed them that he would not be flying today, and given him a phone so that he could discuss the situation with his boss and his employer. They too could have rationally talked the guy off the plane, or alternatively they could have given United information that would lead to letting the guy fly.