I don't think Obama had a ghost writer (I could be wrong). He certainly had an editor, as every writer does, who probably had a substantial influence on the book, but that's a lot different than a ghostwriter. Editors strengthen and clarify the intended message of the author. They do shape and influence the narrative voice, but they do not create it.
With a ghostwriter, the so called "author" basically just talks to the GW in interviews and provides facts, then goes along their merry way. It is the GW who does all the actual writing, creates the narrative voice, etc. The subject matter might be Palin's life, but the book does not belong to her. Hell, I wouldn't be surprised if she hasn't even read the manuscript all the way through.
Ghostwriting should be more openly criticized as the sham art form that it is. If you can't write your own book, don't write to begin with. I refuse to buy any book that is written by a ghostwriter.