We've got 15 years to go. He died in '58. For me the question is why is St. Paul and Epicurus, essentially the sequel to Epicurus and his Philosophy (or rather EAHP is the prequel to SPAE from his perspective), in the public domain but not EAHP?
That might be answered in the opening pages of the originals (sometimes authors leave a work in the public doman intentionally, and usually they do this in their opening pages).
Otherwise, I can contact either "Lawyers for the Creative Arts" (they legally advised me when I signed the Humanist Press book contract) or I have lawyers next door to my office where I work that focus on copyright law. I could drop in and quickly ask them one day. This does sound shady.