Actually it looks like they did do something wrong. According to what I’m reading elsewhere United has the right to bump passengers BEFORE they board the plane but not AFTER they board the plane.
They can dump them inflight if they want.
Rule 21 Refusal of TransportUnited Airlines Contract of Carriage
UA shall have the right to refuse to transport or shall have the right to remove from the aircraft at any point, any Passenger for the following reasons: ...
In the end, they bumped the good doctor not because they overbooked, or they had to seat a United crew.
They booted him out because he disobeyed legal instructions from the flight crew and became disruptive.
IMO the good doctor was no better than an unruly drunk that creates a scene and is also removed.
The fact that he was beat up was on him.
As I learned on another thread, they actually do, and it is all codified in US Law. I too was outraged until I learned more.