You are correct and my post was not precisely worded. I was not trying to say it was OK to forcibly remove a boarded passenger for this reason.

I was responding mostly to the claim that they can only denying boarding in the event of an oversale because they can deny boarding to accommodate positioning crew, it happens all the time.

I agree that once the passenger was boarded, the situation was different. United screwed up, and they compounded the problem in their violent removal of the passenger and their subsequent public responses to the incident.