And then you can _actually_ begin to articulate why someone is culpable and another person isn't. A magical free will believer can't really make sense of that, because if free will is something indescribable and magic, there is no reason to believe a person does not have free will to do X, just because they e.g. are incapable of doing it.
Conversely, if you believe _both_ in magical free will, and have also thought about the material conditions for free will (e.g. they must be informed, and understand the consequences, and not be coerced etc.), then you already have a completely coherent materialistic belief about free will, and have just awkwardly bolted supernaturalness to its side, completely unnecessarily.