This didn't occur by accident, force was called for and applied to a contractual partner of UA, subsequent to a contract dispute.
Captain's mistake was calling/authorizing call for law enforcement - ie. calling for force. He had other options among which refusing the United deadheaders, or calling a mech malfunction and reboarding after emptying the plane. Ultimately the Captain is the authority who demanded that force be applied to this passenger.
The rent-a-cops may get fired because they stepped into a civil dispute outside their remit, although their job profile should allow them to say "we followed orders". The Captain is supposed to be smart enough to understand the consequences of unleashing gorillas on a physician - he deserves to be blacklisted by all US hospitals, exactly like a dsiruptive passnger would be placed on a no-fly list.
As for United's President, he certainly made a case for most jobs being done equally well -equally badly- by robots
