I will allow the fact that airlines do give themselves get out of jail free cards in the contract. They can do what they want, when they want and don't have to tell you at all. That is true.

Second point, the article said that these were 4 people from a partner airline. I take that as meaning that it is another airline under the same corporate umbrella as United, otherwise why would you do this for people with no financial or economic ties to you?

The problem that the airline had was they needed to get 4 ppl form Chi to Louisville in time for a Monday flight. The airlines function in its simplest form is to get people from point a to b in an efficient manner. They know that these people have to get to Louisville. They know they have to get there from another location that is probably not initially chicago. There had to be contingency plans in place in case something like this happened. All of that aside, passengers don't just materialize at the gate, you scan your boarding pass and alert the airline you are there. There was a point in time in which the airline knew that all of the seats were accounted for including people who were making connecting flights into Chicago.

Whats interesting to think about is knowing that all the seats are taken and you are going to have to convince someone to give there seat up, one flight is going to be delayed regardless. What if they delayed one of the connecting flights long enough that people just missed the flight to Louisville? Happens all the time, it is a risk that everyone who travels that way understands and is willing to take, no one would have been the wiser, and it avoids all this bullshite.