You don't see moderates behaving that way towards moderates.

Presumably by a "moderate" you mean someone who agrees that "politics is a mind-killer", not a DNC supporter, because those are not much better.

So people are not defending O'Reilly's book because they have a bias that makes them agree with him about the content of the book.

Of course not, for many of them O'Reilly can do no wrong, so anyone who attacks the book attacks their values. The content of the book is not relevant, only the fact that Bill O'Reilly wrote it is.