It's all part of the culture. There are many "authors" besides talk show hosts who are more-or-less guaranteed to get on the best-seller list on their credentials as "celebrities'" (which automatically imply media exposure) rather than because of the value of what they have to say - and they usually don't write the stuff themselves. Also, in the NY Times at least, best-sellers are marked with a dagger if booksellers report bulk sales, usually indicating that groups affiliated formally or ideologically with the author buy up copies to give away for the sole purpose of inflating a book's supposed "popularity".