The well-informed discussion on Naked Capital (http://www.nakedcapitalism.com/2017/04/united-passenger-remo...) strongly supports the claim that the removal was illegal. It also provides interesting evidence that the decisions were traceable to United's attempt to reduce costs by using third party contractors and affiliates (the flight was operated by Republic, an affiliate).
This quote is telling: "When I was a outsourced regional jet captain operating United Express flights between 2010 and 2014 the gate agents in charge of the regional (out-sourced) flights at United hub, Washington-Dulles, were third-party contractors. They were horribly trained and frequently surly. The gates were always crowded, everyone there was angry, nothing worked, it was utter chaos and misery. I absolutely dreaded flying there and did my best to avoid it, Chicago was only a little better."
This quote is telling: "When I was a outsourced regional jet captain operating United Express flights between 2010 and 2014 the gate agents in charge of the regional (out-sourced) flights at United hub, Washington-Dulles, were third-party contractors. They were horribly trained and frequently surly. The gates were always crowded, everyone there was angry, nothing worked, it was utter chaos and misery. I absolutely dreaded flying there and did my best to avoid it, Chicago was only a little better."