Tolstoy's point is that history also has to get it through its thick skull that there’s no such thing as free will, and that every person is limited in the kinds of things he can and will do in a given situation.
In every other kind of examination of how and why people do what they do, we take these limitations on available actions into consideration. For instance, a man who kills is allowed to explain why he did it. When we see that his choice of action was really limited – for example because he had to kill in self-defense or because he was a soldier – then we attribute a lot less guilt to him. So why not in history?