I have never said the phrase "fine print" once, so I'm not sure why people keep referring back to that, as if I am using it as some kind of excuse. The airline has every legal right to bump you from a flight. It happens to people every single day. However, the airline cannot forcibly remove a passenger who refuses to get off of the plane. And I'm not missing the point, you are:
There are many different entities involved in a flight: The airline provides the plane, books the tickets, processes the check-in, etc. The airport provides the mechanism(s) for conveying bags from the ticket counter to what they call the makeup unit, which is where either the airline or a contractor will collect the bags for a flight, put them on a cart, or multiple carts, and take them to the airplane and load them. TSA provides the screening of the baggage for explosives, in almost all airports. The airport police department or security contractor will handle all physical altercations required in the airport. United(by the way, it wasn't even United, it was Republic Airlines operating a United flight) followed procedure that is used at virtually every airport in the country. If you have an unruly passenger, or a passenger who refuses to follow the flight crew's instructions; you don't touch them. You call the cops. Once the cops show up, it is out of your hands. Whatever follows, in terms of how the situation is handled, is on the cops. United doesn't have a representative standing there directing the cops, "Grab him!" "Now armbar him" "TASE him!' "A little more pressure on the clavicle. There ya go." As it turns out, because it was Republic airlines, a contract carrier for United, that operated the flight; United's corporate policies limited the amount of money that they could offer to induce people to give up their seat. I would bet that they are already in the process of reviewing those policies. But the bottom line is: The people on the ground did what they could to entice passengers to give up their seat. When they didn't get enough takers, they were perfectly within their legal rights to randomly select someone to bump from the flight. When that person refused to leave the aircraft, they followed the well-established protocol, nationwide, of calling the LEOs to remove the person. Again, this is similar to you calling 911 to get the cops to break up a fight next door, then the cops showing up and shooting one of the combatants, and people blaming you for the guy getting shot. You didn't ask the cops to come shoot anyone, and you did what was reasonable, in asking for the appropriate authorities to come resolve the issue. If those authorities over-react, that's on them; not you.