I worked for United 1968-2003. I'm not defending them but here are the facts as I see them;
1) Overbooking is a policy that started when the airlines were government regulated by the CAB. The CAB went away in 1979. At the time the airlines argued that they were losing too much from no-shows, the fares were regulated, and there was no way to recover the loss. Fair enough. Now there are penalties for being a no show, rules, re-booking fees etc,etc., so maybe overbooking has outlived its usefulness? By the way JetBlue has a policy of NOT overbooking-just sayin!
2) That wasn't really United BUT United Express, a group of contract carriers who pay their people dirt, no benefits, are under trained/supervised and have no sincere interest in seeing the job done right. So United lets them dress their agents, and paint their planes while thinking they save money on such a deal. United will take the fall, and maybe they are getting what they deserve? If it looks like United, it must be United.....not true! Even though this happened at ORD, a UA hub, UA does not handle the UA Eexpress flights, even at ORD. Having said that, once you get away from the hubs, all the employees are contract people. My wife & I, retired from Seattle but live in the Richmond VA area. We have travel benefits by virtue of my 35 years but given the choice, we drive to Dulles to fly. The contract operations are too painful to watch or experience. That's only because we are both UA retirees and we, despite being screwed out of our pensions, do still care!
3) The reason they needed the seat was for a dead heading crew member, who was likely needed in Louisville to cover another flight. That's NO excuse but unfortunately is the fact. IF they were compelled to ride a jump seat, it would count against their crew rest time, which bites into their duty total duty time available out of Louisville. (FAA duty regulations). The United Express crew desk needs to get their act together so that crew are moved around the system without passenger disruption or notice!
4) The Police officers were not Chicago Police but TSA/Homeland security people. I worked at O'Hare in the 80's, and the Chicago Police had the Airport security responsibility but that was well before all the TSA theatrei
Sadly people have bad experiences on ALL of the airlines. There are a lot of people who would NEVER FLY this or that carrier. Fair enough, it's a free market place, but the over booking policies of ALL the airlines need to be done away with. They are no longer economically justified in any airline business model..