Originally Posted by Aviatrix
But I don't understand why this is now being enforced for "security" reasons. How exactly is security being jeopardised by allowing people to venture into the other cabin? Could someone please explain this to this ignorant European, preferably in words of one syllable?

Well, see, terrorists are rule-followers. If you tell them they can't go from coach to first class, they can't get to the cockpit.
Of course, if they buy first class tickets, then they could access the cockpit without crossing through another cabin. And those of us do-gooders in business/coach would be powerless to help, as crossing the cabin divider to stop the terrorists would be a violation of crewmember instructions, which is a federal offense.