flapsforty
Agree with you. Talking pre 9/11, I usually relied on the judgement of the purser as to the frame of mind of the passenger, as they have direct contact, and their people skills are usually very good.
In two instances I allowed ground staff to sway me by reporting that they had spoken with the passenger in question, and they had promised no more disruptive behaviour. In both cases, soon after airborne, fights broke out in cabin. (One over smoking in the bad old days of smoking. The other over use of seat recline.) It is amazing how quickly one disruptive person can change dozens of seemingly normal, amiable, passengers into hooligans.
Having had a number of scares I tend to be conservative now, but insist that the utmost tact, and maximum number of impartial witnesses, be used.
Post 9/11 the problem has assumed even greater complexity, as the definition of "non-normal" behaviour has been broadened immeasurably.
I am absolutely not any kind of lawyer so I can only view the subject from the point of view of a normal and reasonably prudent person.My view is that in these times of heightened sensitivity, the litmus test must be some sort of demonstrable "non-normal" behaviour, including failure to follow reasonable instructions.
One would think that a reasonable person would comply after one warning, and that anybody who failed to comply after such warning could be deemed to be a "risk", and subject to offload.
As usual the utmost tact, and a goodly supply of impartial witnesses would be desirable.
In fact, this would probably be safest for the "questionable" passenger too, because trigger happy passengers these days are not going to tolerate any non-normal inflight behaviour.