If you are in the air and consider that someone is acting illegally the Captain can take any reasonable action to protect the pax, crew and aircraft.
If you are on the ground and think a passenger disembarking may be illegal, you must call the local police as any action by you could be seen to deny the individual of their rights and you could be personally liable.
The bottom line of course of the original memo ( if accurate ) is cost not security or any other issue.