As a reader-participant-watchdog-community member-content co-creator notes, grouping these with the cookbooks and marriage manuals has the effect of shifting these off the main best-seller shelves at Barnes & Noble and Borders (what remains of it). If these books were included on the regular nonfiction list, some of them would probably be quite competitive. On the USA Today list, which lumps everything together, including fiction, Mr. Luntz's book (which, in fairness, does seem like more of an advice/how to book than the other two) is at 68, above Stacy Schiff's Cleopatra, which is at no. 3 on the New York Times nonfiction bestseller list but down at 90 on the USA Today list.
I have to admit I don't know much about the process by which a book gets categorized as one or the other; in some cases it's possible that a publisher would ask to to be in the "advice and miscellaneous" category if the publisher somehow thought the book had a better chance of breaking out that way. If I had more time to report it out with the publishers, the authors, and the Times Book Review, I would, but I don't, so over to you, Michael Cader.