Sorry, but that's hyperbole. I'm fairly confident the interaction didn't go down as: "Hello, we have an uncompliant passenger refusing to leave the aircraft at gate XYZ. Please come down and beat the shit out of him for us, thanks!"
>Law enforcement got involved in a civil dispute between a customer and company and then battered that customer on behalf of that company.
Ignoring the continued hyperbole, the dispute became more than a civil dispute once he refused flight crew instructions to leave the aircraft.
>There was no safety or security justification for law enforcement to be involved at all.
There was once a passenger demonstrated they're not willing to comply with the flight crew. Contrary to popular belief, commercial aircraft are not glorified busses.
>The flight was delayed due to United's actions (kicking passengers off to board crew, after the flight had already been seated).
Yes, but his actions then delayed the flight much more than it otherwise would have been. A minor delay due to crew positioning is a small price to pay if it means not having to cancel an entire flight of passengers down line.
>The passenger was asserting their contractual and legal rights, ...
Then argue that to your heart's content after deplaning. The aircraft you're being told to leave isn't the proper forum for legal and contractual disputes.