In my high school, there were several teachers like this. It was extremely frustrating. However, so long as he's still willing to give As for A work, it won't affect her GPA. While I don't completely agree with the policy, I do understand it. And I also know that the teachers with this policy were the ones who got the most effort out of me. If one of them had told me that it was one of the best essays he'd ever read, I would have glowed for a week. That would have meant so much more than the grade.
ETA: I also agree with others that this will just continue to happen. You don't have to like it or agree with it, but it would go better for everyone if you both got used to the idea.