I discovered that it has less to do with outright evidence and more to do with distrusting the sources that tell you otherwise. The reasoning goes, "If they lied about this, then they lie about everything." The problem is that this sort of thinking quickly becomes isolationism, or, "you disagree because you are evil too". I empathize with the movement and agree with many of their conclusions but I believe a lot of it is simply hate fuel. Everyone wants there to be a clearly defined bad guy in the story they are protagonist in. Its simply not that simple.