Copyright infringement?
Was thinking the same thing myself.
Intellectual Property law can be wildly complicated. There's quite a bit of case law on the subject. It get's well into principles such as satire and parody and the fair use doctrine. They don't "copy" any of Palin's words, or pictures, but there is a striking similarity.
Even if they did, there's a special rule for Artistic Works, to include literary titles. Courts have given people great latitude in such cases where people are either satirizing, parodying or making a political commentary on other literary works, and I assume the same would apply here. The long and short of it is copyright law isn't nearly as protective of Titles and covers as it is of the book's content. In my estimation, an infringement suit would not prevail.