Right. So the 'security' aspect of checking the BP again at L1 is to identify those passengers who have materialised or dematerialised in the jetway ?
Hey, don't expect me to defend the security procedures in place. We do what the DFT ask of us. Allot of it is barking at best. I suspect that all that is achieved at the door to the aircraft is to allow the cabin crew to direct you to your correct seat, and correct cabin.
Just on my most recent flight a passenger sat in First class, with a coach ticket, and refused to move. Full stop refused. Only with the threat of the Police did he go to his correct seat.