One option that I considered that Cotton didn't mention was that it was actually an accident that those reviews were included in final printing, after maybe being in drafts as a spoof or a joke or whatever, and accidentally not having been removed.

I'm telling you this fascinating story because I just had reason to ask these questions myself. I read the fabulous Harper's Magazine and this month's issue includes an ad for the new book by Jonathan Franzen, The Discomfort Zone. Is the text in the image too small to read? The second review says, "[A] total lack of humor... perverse" and the fourth says, "Odious... incredibly annoying."
So, what's the deal? Is it a gimmick to catch my attention (it did)? Is it an inside joke style review, given the title and subject of the book? My theory of the mistaken inclusion is ruled out by the fact of this being one small clear ad, not pages of filler reviews that no one reads. Why'd they do it? Was it Franzen's idea? More importantly, have you bought me this book yet? I loved The Corrections.
[0] From the poem "Sonnet for Minimalists" by Mona Van Duyn.