The whole thing is a bit of a half-measure. Even if people's actions are predetermined, and they are not morally accountable for them, we need to hang them anyways because without such an incentive, even more people may be billiard-balled into doing the same things.

Of course, this is all quite besides the point. If you swallow billiard-ball determinism hook, line, and sinker, there's really no point in talking about why we do anything, though, as it works out, there's also little point in objecting to people discussing why we do anything, since it's all going to happen just as it happens no matter what.

ETA: I'm referring to the commonly misconceived notion of determinism that thinks free will cannot exist because the universe is "merely" physical. I mean "billiard-ball determinism" as something of a pejorative, not as an accurate model of how the universe really works. I'm not claiming that a deterministic universe is incompatible with free will; indeed, I believe the opposite.