On what charges? From my (and other's) reading of the contract of carriage, he was absolutely within his rights to refuse to give up his seat. The police would have had no legal basis for arresting him.
Have we come to the point that we are willing to let the government and corporations have arbitrary power over us, without any basis in law? Even though the airlines have bought off their captured regulators to the point that the passenger almost no power whatsoever in the terms of the contract of the ticket, with a regular part of their standard operating procedures such that in almost any other business, would be prosecuted as outright fraud, yet they were still on the wrong side of that slanted 'contract', which is barely worthy of the name.
‘From my (and other’s) reading of the contract of carriage, he was absolutely within his rights to refuse to give up his seat.’
The CEO of United agrees with you.