“Dr. Dao certainly had a license to be where he was. “
No, he didn’t. His boarding pass was revoked.
Explain this ‘license’ you speak of that didn’t exist.
No need to. United's future actions and the settlement they pay will say all that needs to be said about the situation.
And "revoking" somebody's boarding pass for no valid reason doesn't erase the rights they had under their contract with the airline, and FAA regulations.