You're imagining a baseline of reasonableness. Humans have competing preferences, we never just want "one thing", and as a social species we always at least _somewhat_ value the opinions of those around us. The point is to imagine a system that values humans at zero: not positive, not negative.
Still there are much more efficient ways to extract iron than from human blood. If that was the case humans would have already used this technique to extract iron from the blood of other animals.
