It's a popular thing they teach in college nowadays (not the D-Day thing -- the "how can you really know anything" thing).
Ann Coulter's books are heavily researched and footnoted. When she says something, its verifiable. When michael Moore says something, he is expressing a viewpoint which is (as he says) as much designed for "comedy" as it is for truth.
So, yes, we cannot escape the author's point of view, but some people try hard to have a view that reflects the truth, and others just want to convince people of their viewpoint.
Actually, there are plenty of sources that Ann doesn't quote because they don't support her point of view. The ones she quotes are verifiable, but you don't have any way of knowing from her bibliography or her notes what is out there that disproves her points.