Sure it was not "overbooked" in that strict sense, and sure he had already boarded. And heck, sure, he wasn't offered "maximum" compensation. I can accept all that. 

However: United didn't do anything that they aren't expressly permitted to do, by the contract that all ticket buyers sign. Contrary to common opinion, buying a ticket does not guarantee you a seat and never has. That is part of the contract. 

And I would repeat: once he was asked to get off, as unfair as it was, he brought the rest upon himself when he refused over and over. I see his refusals as a deliberate attempt to make himself into a martyr. He knew full well the plane wasn't going to leave with him on it and yet he kept refusing to leave, forcing United to physically remove him (and boy did he scream and wail for that, notably). This is why the man's criminal history is relevant. I see this as a case of United being a bit unfair to a customer, who happened to be an opportunistic fraudster.