If he sticks to his original commitment--no polar bear attacks, no way--can we really say that he is doing his job? Now, lets suppose that a polar bear is actually to blame. It escaped from the cargo ship and ran into the unsuspecting victim before wandering back to the dock. We know this, but he doesn't. In this scenario, not only would he not be doing his job, but he would be searching high and low looking for any explanation other than the truth.
You should immediately see the obvious problem with the above scenario. Granted, I have loaded it to make my point obvious. Even so, our forensic investigator should not rule out any causes of death, no matter how unlikely they may seem to him.
Why then is it often considered virtuous for scientists and historians to rule out the possibility of God and/or supernatural events before they even look at the evidence? They set their mind firmly against it. It could never be. What do they do when all of the evidence clearly pointing to an intelligent, transcendent creator, or a supernatural event? Evidence like specific and purposeful complexity, the incredible fine tuning of the universe for life on Earth, the laws of thermodynamics, the implications of a universe that has a definite beginning and so forth. Evidence like an empty tomb and the totally transformed lives of incredibly unlikely people who stood to gain nothing from perpetrating a conspiracy. That raises the question; if one is forced to reject evidence in order to uphold his premise, is he really following science or history as he claims? Is he really doing his job? No. In reality, he is bound to a theological position (albeit a negative one) just like the theist he mocks. He accuses the believer of ignorantly dismissing any evidence that seems to be against God and/or the supernatural while at the same time dismissing all evidence that seems to be for the existence of God and/or a supernatural event. In such a scenario, it isn't dogma versus reason, but dogma versus dogma. If we cannot follow the evidence where it leads--regardless of where it leads--reason has been thrown out.