It's a matter of intent.
Plagiarism is intending to pass off someone else's work as your own.
Usefully, the History News Network has gathered together the definitions of plagiarism offered by three organizations: the American Historical Association, the Modern Language Association, and the American Psychological Association. All of them speak to intent. Since the example at hand involves a historian, let's start there.
The word plagiarism derives from Latin roots: plagiarius, an abductor, and plagiare, to steal. The expropriation of another author's text, and the presentation of it as one's own, constitutes plagiarism and is a serious violation of the ethics of scholarship.
Plagiarism includes more subtle and perhaps more pernicious abuses than simply expropriating the exact wording of another author without attribution. Plagiarism also includes the limited borrowing, without attribution, of another person's distinctive and significant research findings, hypotheses, theories, rhetorical strategies, or interpretations, or an extended borrowing even with attribution. Of course, historical knowledge is cumulative, and thus in some contexts--such as textbooks, encyclopedia articles, or broad syntheses--the form of attribution, and the permissible extent of dependence on prior scholarship, citation and other forms of attribution will differ from what is expected in more limited monographs. What belongs to whom becomes less distinct. But even in textbooks a historian should acknowledge the sources of recent or distinctive findings and interpretations, those not yet a part of the common understanding of the profession, and should never simply borrow and rephrase the findings of other scholars.
Plagiarism is a broader concept than that of a legally protected property right or copyright, the AHA notes. Plagiarism works "harm . . . to the pursuit of truth." Accordingly, "All who participate in the community of inquiry, as amateurs or as professionals, as students or as established historians, have an obligation to oppose deception."
The MLA also focuses on intent:
"In short, to plagiarize is to give the impression that you have written or thought something that you have in fact borrowed from someone else."
And so does the APA:
The key element of this principle is that an author does not present the work of another as if it were his or her own work.
Historians of all people should understand the obligation to the fidelity of one's sources, and yet there exists something of "a crisis of confidence in those who are stewards of our national history." So writes Ralph E. Luker in reviewing three books on the subject. Maybe within their covers lies the answer to why this problem persists. I don't know.