That is the question posed in this article from Slate. It is a fairly interesting article on how all of these reviews of the memoir were posted so quickly. Normally, reviewers are given advance copies well before the publication date. Shoot, i've even recieved a couple of advance copies because of my Amazon.com reviews Which reminds me, i should get back to work on reading Alexander Hamilton. It isn't very advance anymore. Whoops. But back to my main point. The publishers of Clinton's book, Knopf, decided that there would be no advance copies released until the day of publication...which makes one wonder how some of these reviews were turned out in 24 hours. Even speed readers would have trouble with that one. This article explains how it all went down.
Side note: While i didn't expect one because i'm not actually employed as a reviewer and haven't yet (key word) attempted to seek employment as a reviewer, i would have loved an advanced copy of My Life.