UPDATE: "It turned out this whole story has nothing to do with Egypt or Holocaust. Historian claimed that he would give a speech for money. Incompetent CBP officer incorrectly concluded that its a work without a proper work visa."
It seems the decision by the officer was actually upheld by his/her direct superior (referring to the original. Translation: "Not being in possession of a federal document authorizing me to work in the United States, I am in violation. The decision will be confirmed later by his superior - that I will not have the opportunity to meet.").
It's more subtle than that. If it hadn't been for the very specific exception for scholars it would indeed have been work without a proper work visa.