Purchasing a ticket creates a contract between the individual and the airline; so long as the passenger upholds their end, which from what I can tell this passenger did, the airline is violating their end of the contract, and worse is getting the police involved in what is essentially a contractual matter. This is dangerously totalitarian, getting officers who are employed to protect the population to enforce the whims of the corporation. I simply cannot believe anyone along the chain of communication here didn't stop and say, 'this is wrong'. The lack of humanity is simply astounding - I am at a loss for words.
I sincerely hope he sues both the airline and the police for this, it's assault and breach of contract. I'll echo what others are saying, this is a dangerous situation to allow to quiet down. We need to keep pressure on United until the responsible parties are made to answer for their decisions.