Any of these readers would have been willing to believe that Bill O'Reilly had written a bad book, if they did not believe that Bill O'Reilly was strongly biased.

Are you possibly confusing what these readers said with what they believe? I suspect many of these people had no well-founded opinion on the book, or may have privately thought it was a bad book, rather they were seeking to defend the author for political reasons.

So in this book review example, it's just that people who have a strong affiliation with a well known political figure will seek to defend him regardless. When we read messages like this, they tell us practically nothing about what the defenders really think about the book ( probably in this case they haven't even read it, there is nothing to tell ).

[ My first comment on this site, so be gentle - I'm just getting acquainted with the furniture, so I may well be wide of the mark. Reading further down, it seems others are thinking on similar lines (TheOtherDave), this was my reaction before I read that. ]