I think that the attitude of the documentary photographers claiming objectivity is generally a bit ingenuous and naive or, on the other hand, the result of a rhetoric strategy. They might be honest, but not objective. I think they manage to manipulate these kind of photos later because they photograph what is happening in front of their eyes. Joan Good documentary photographers are perfectly aware the way they filter their vision of the world. They are also aware that they are not only giving testimony but also spreading propaganda.
Every photograph tries to influence the conduct of the others.
When I take a photo I don't do it for free. If I take a photo of someone who is starving, it's to report this situation. So at its core, what that photograph wants to accomplish is a movement of the sensitivity of those who can do something to ameliorate this disaster. I have a purpose and a medium to fight for this cause. The great model of the documentary photographer is Eugene Smith , who did for example, the famous series of photos of Minamata.