According to a CBS article, the US has a policy that no one single person can be in the cockpit alone during a flight. If one of the pilots needs to leave the cockpit, a member of the flight crew will step in until the other pilot returns.
Apparently this is not the case in Europe. Perhaps it will be now.
How unfortunate this happened.
According to the Telegraph:
"[Carsten Spohr, CEO of Lufthansa,] said that in the US there is a rule that a steward remains in the cockpit when a pilot leaves, but that this is not the case in Europe and that he does not think it is necessary to change the procedures, despite the tragedy. "
Penny pinching CEO sticking to the low cost line no matter what.