It was the Chicago Airport Police which dragged him off using force, not the airline.
The person who did the physical removal was not dressed as a policeman, he was wearing jeans and what looked like a security guards uniform, could have been employed by UA or a subcontractor to UA, there are several possibilities.
Regarding the 'property' aspect, you haven't invited this person on, they have entered into a business agreement with you and you have sold them the use of a seat for a specific flight, that comes with contractual responsibilities and liabilities and unless that passenger contravenes specified regulations then you, the owner, don't have just cause to throw him off, literally. What you, the owner of the seat, can do is offer to buy back the seat you sold him, for an agreed price.