Why would it be necessary to remove passengers already seated on the plane? If they were seated at the time of the breach, they cannot logically have committed the breach and would therefore be secure? Or did I miss something?
Because if they came into contact with the person who returned back into the sterile area, they may have been passed items which are not permitted on aircraft. It makes perfect sense, even if it's hideous inconvenient.
The dumbest one of these is the re-screening of transit passengers in Australia. You get screened, board a secure plane, fly at 35,000 feet for 3 hours and then get screened again immediately you get off. Exactly where do they think you would have got hold of something you shouldn't in that flight?!
Because you mix with passengers from any other international destination, including those who have lower security standards. In theory, you could have come into contact with someone who originated from a country with lax security standards and been given contraband. At security screening, they also don't know where you're from.
New Zealand, incidentally, also re-screens transit passengers. Your beloved United Kingdom even does.
Not if I board an otherwise empty plane originating in Auckland, then fly to, say, Brisbane. Everyone boarding in Auckland would be screened in Auckland. Then you get off and are immediately screened again, before meeting any other passengers.