It appears that the flight was NOT overbooked. The "guests" were booted (and beaten) to accommodate airline employees. Poor planning on United's part will cost them dearly, and a rent-a-jackboot, or three, their job(s). Rightfully so.
The Captain of the flight has been given authority over the flight in order to safely operate it. At some point in flying, you have to put your faith in the abilities of the Captain. And as an airline captain, if a passenger demonstrates he is unwilling to comply with crew member instructions, I have very few options except to have him removed from the flight. Passengers that are unwilling to comply endanger themselves, other passengers, crew members, and others. E.g. if its necessary to evacuate the airplane, there just isn't enough time to discuss whether or not you can grab your crap out of the overhead before exiting.
United did not have a choice about who to call to remove the passenger. Security at the airport is a combination of the Airport Authority, TSA, and local law enforcement. The Company doesn't even have the authority to overrule the Captain. If the Captain deems the passenger unsafe to fly, FAA says he can't go.
The options are clear. Plan accordingly to get your crews to where they need to be and not inconvenience paying passengers who are already seated and already have baggage loaded and gong to your destination. Airlines, and their demi-god employees, have thoroughly ruined air travel within the USA. Airlines treat their "customers" like crap, allow and encourage perverts to fondle old ladies, cripples and children in the name of "security" and give 'authoritah' to the incompetent to remove anyone for whatever reason that can be conjured from a fevered brain.
Poke a lion in his cage enough and he'll try to rip your head off. This flight wasn't even overbooked. Four special people were granted special rights to toss passengers off so their lives could be made easier.
If the four special folk were delayed getting to Louisville and a flight had to be delayed, that responsibility would fall on United. Their stupidity, their problem.