I think there is a whole load of factors and think you will find that majority of it will relate to what Captain will allow.
The problem with having a hard and fast rule is that it negates the Captain having any discretion.
Just because it happened at X Airport with A Airline doesn't mean at Y Airport with B Airline it will be the same.......
Crikey if the airports could get security standardised first that affects majority of passengers then thinking of a policy that affects few people might get a look in.