The US has had a sterile flight deck policy for decades. It didn't prevent four hijacks in one day in 2001. Lack of security on the ground let those blokes (and there were NINETEEN of them) get on.
Much of the civilised world, on the other hand, didn't have such blanket rules prior to September 2001 and we managed to operate safely up to, on and after that date.
There are still some places where it's allowed - just ask the cabin crew chief nicely when you step aboard, explain your link to aviation and if it's not banned by the airline's country, in my experience crew pretty much always pass the request onto the Boss and let him/her make the call.
As it should be.