I have no idea what the flight crew knew, however if I was the Captain and knew how this lady was behaving before we departed I would have had her removed.
And thats the issue, the captain is highly unlikely to have met or the seen the passenger so can only go on what he/she is told.
Pain in the a*rse and rude passengers are standard fare, disruptive to the point of have to be removed much rarer. In practice it's really the cabin crew who are left to make the assessment and the captain merely act on thier report/recommendation.
In this scenario its possible that the CC opinion of how disruptive the person was differs to yours.