Several commenters have posted the relevant section of the "Carriage Agreement":
Aside from the fact that no one (well, almost no one) ever reads the carriage agreement, there seem to be a few problems here:"4. Compensation for Passengers Denied Boarding Involuntarily
a.For passengers traveling in interstate transportation between points within the United States, subject to the EXCEPTIONS in section d) below, UA shall pay compensation to Passengers denied boarding involuntarily from an Oversold Flight at the rate of 200% of the fare to the Passenger's first Stopover or, if none, Destination, with a maximum of 675 USD if UA offers Alternate Transportation that, at the time the arrangement is made, is planned to arrive at the Passenger's Destination or first Stopover more than one hour but less than two hours after the planned arrival time of the Passenger's original flight. If UA offers Alternate Transportation that, at the time the arrangement is made, is planned to arrive at the Passenger's Destination or first Stopover more than two hours after the planned arrival time of the Passenger's original flight, UA shall pay compensation to Passengers denied boarding involuntarily from an Oversold Flight at the rate of 400% of the fare to the Passenger's first Stopover or, if none, Destination with a maximum of 1350 USD. "
1) The guy was already on the plane. IOW, he had not been 'denied boarding'.
2) The compensation offered was in the form of a voucher. I am not sure this can be considered a USD payment.
3) The people who were being given the seats in question were UA employees, who were apparently needed as flight crew at the destination airport. Can this be counted in terms of the flight being 'oversold'? (i.e. They never bought a ticket.)
The police involvement in what is essentially a contractual dispute where no one's safety was at risk is questionable, unless the man's continued presence on the flight was a 'trespass'.