Darwinists seem to think that speaking honestly and freely should only be done for friendlies. And that if documentary makers interview you they have to agree with you. Otherwise it's no fair!!
Besides, it is doubtful that the film makers even do disagree with the historians on the subject matter of the historical Darwin. If they disagree on the actual correctness of Darwinism as science, well, what does that have to do with the historians, anyway?
Also, from a comment to the post:
It is complete nonsense to suggest that a journalist is required to tell people whom he interviews what the perspective of his documentary will be. If such a stupid code existed it would inhibit the ability of a journalist to gather basic facts.
Furthermore, it is ridiculous for someone who testifies in documentary to complain that they would have given different testimony had they known the documentary was going to be against Darwinism, as if there is one set of facts that one discloses when speaking with Darwinists and another set of facts when speaking with skeptics of Darwinism.
The very fact that witnesses are complaining that they would have testified differently had they known the perspective of the documentary demonstrates the very need of the journalist not to reveal his or her perspective in order to get unbiased testimony.