The defining issue here is (as I've written about many times before) the cellphone video from passengers who recorded the event. It sears the message via our eyeballs right into our brains. Columbine, 9/11 and the towers collapsing, and the images from Katrina. Give me this same story of an incident on a plane, with no video, and it would have likely been a local news event, if that.
The big mistake was made at the gate. I've sat in airports where the flight is overbooked and the gate agents keep sweetening the deal until someone takes it. Offer up, a round-trip ticket; round-trip ticket and $500; first-class ticket; first-class ticket and a one-night stay in the White House with President Trump, etc. The fault could be that the local management put a clamp on those types of offers. I bet they wish they could have offered a free flight and $10,000 in return for no negative publicity.
Lastl the first response from United Airlines was "tone deaf" to the video. When caught in a compromising situation — apologize, don't dig in, even if you think you are right. The video is the message, the facts of the matter don't carry as much weight as the video.