Actually, his handling of the topic "plagiarism" for the AHA was quite good during his tenure as ethics watchdog, as can be read here and here. Hoffer left the AHA some time after it decided it would no longer investigate ethics charges against its members. (I assume his leaving was connected to that.)
I wonder how many members of the AHA have now or ever complied with this old Hoffer guideline (my emphasis added):
In print, all paraphrases, no matter how long or how many works are paraphrased, must be followed by citations to the sources that are as clear and precise as those provided for a direct quotation. The citation should refer to the exact page(s) from which the material was taken, rather than a block of pages or a list of pages containing the material somewhere.
We saw in our review of Stephen Sears' citational habits that he is a major offender on this point. Here's an additional rule that Sears needs to read twice:
All works an author consults should be either cited in the reference apparatus or in the bibliography. If particular pages were consulted, these should appear in references. By contrast, works not consulted by the author, even though they may be relevant to the topic, should not be cited. Such a citation would give the false impression that the author had used the work.
I'm thinking in particular of his Young Napoleon which had a "works consulted" list filled with uncited books arguing directly against Sears' surmises and conclusions - without mention of their content in his text.
Unfortunately, pop historians are given much looser rein in this obsolete AHA rulebook:
What is generally termed popular history ... rarely conforms to the same standards of citation as scholarly monographs and interpretive essays. Many popular histories, for example, have only a short list of works consulted. But wholesale borrowing from another work, even with attribution, is unacceptable. Ideas themselves cannot be plagiarized, but authors may not claim as their own the full-dress presentation, according to the AHA Statement on Standards, of "another person's distinctive and significant research findings, hypotheses, theories, rhetorical strategies, or interpretations."
As you make your way through whatever bestselling historical blockbuster, remember that these authors are accountable to you, the reader, for evidence handling and any lapses in integrity. If we, the public, demand higher ethical behavior from bestselling nonfiction authors, we will eventually get it.
(Postscript 1:23 pm: I should point out that the Hoffer material appeared as an opinion piece in AHA's periodical and that the AHA strongly disclaimed any connection with or endorsement of Hoffer's views on ethics or plagiarism. AHA says they publish such articles to promote "wide-ranging conversation." The full AHA disclaimer can be read at the top of each linked article.)