My former debate partner and current Georgetown professor Daniel Nexon has coauthored a book:
I do not know what arrangements, if any, the publisher made with Rowling's publishers. The standard, at least for the Buffy books I read (Buffy the Vampire Slayer and Philosophy is my favorite thus far, though the gender studies one looks interesting) is for the cover to emulate some but not all of the distinctive elements of the source work's trade dress, which you'd think would be taking more than necessary according to the nominative fair use test. In addition, most of the academic studies of single pop culture artifacts I've read don't have disclaimers, even as reviewers label them part of "the canon of Buffy."
(Annoyingly, most of these books have standard ridiculous copyright warnings saying you can't reproduce any part of the text without permission, even when the work relies on repeated, sometimes extended, quotations from the source texts. This isn't irony, it's hypocrisy.)
Copying graphic elements on book covers serves the same function as emulating the shape and color of national brands does for house brands: It's a shortcut, a way to catch the shopper's scanning eye. We could call that initial interest confusion, but it would be better to call it just initial interest.
I do wonder, however, about whether these books are making legitimate use of others' trademarks under non-US law. I know that house brands don't generally imitate national brands in Europe, but I haven't seen any discussion of the relevant legal principles Over There.