'It’s Not Classified Because It’s Marked; It’s Marked Because It’s Classified'
The answer – and the derived authority that authorizes him to mark it – comes from Executive Order 13526, signed by President Barack Obama and issued on December 25, 2009, which says material should be classified when (Section 1.4):
[I]ts unauthorized disclosure could reasonably be expected to cause identifiable or describable damage to the national security in accordance with section 1.2 of this order, and it pertains to one or more of the following:
(a) military plans, weapons systems, or operations;
(b) foreign government information;
(c) intelligence activities (including covert action), intelligence sources or methods, or cryptology;
(d) foreign relations or foreign activities of the United States, including confidential sources;
(e) scientific, technological, or economic matters relating to the national security;
(f) United States Government programs for safeguarding nuclear materials or facilities;
(g) vulnerabilities or capabilities of systems, installations, infrastructures, projects, plans, or protection services relating to the national security; or
(h) the development, production, or use of weapons of mass destruction."
I’ll leave it as an exercise for the interested reader to decide which clause applies to each item on my list of memos, except to note that the fact the leader of the opposition is talking with the ambassador is the sort of thing that could get the leader of the opposition found dead one morning, having committed suicide by shooting himself six times in the back of the head.
The thing is, knowing that the ambassador is meeting with the leader of the opposition isn’t sensitive because someone marked the papers – it’s sensitive because if word got around, it could get someone killed. The stuff from satellite imagery is sensitive because we don’t want people to know how good our imagery is. The cell-phone intercepts are sensitive because we don’t want the Elbonians to know we can listen to their cell phones. But it’s not sensitive because someone marked it – it’s sensitive because of what it is.