My understanding is after 9/11 the TSA somehow disallowed international jumpseating in the cockpit by offline pilots. In the last few years I believe the law has changed, but airlines have been slow to change their procedures. Miami Air is the only one I have heard of that does allow offline cockpit jumpseaters internationally currently. However, about a year ago we had an Atlas guy in Toronto trying to jump to DTW. He insisted he had ridden the jumpseat before but we looked it up and our FOM still didn't allow it. But maybe someone from Atlas can chime in on their op specs.