I was also frustrated by Hill's vagueness on what seemed to be an important point (perhaps he elaborates later?). In any case, I can tell you what I think Hill was thinking when he wrote that, though I'm not exceptionally confident about it.

The concept of human action--of making plans and following through with them--seems to be based on the assumption that the world is fundamentally predictable. We make decisions as if the future can to some extent be determined by a knowledge of the present, paired with a set of well-defined rules.

The natural objection to this would be that human action only presupposes some ability to predict the future, but not the perfect ability that might be possible if causal determinism is true. However, one could argue that it is far more natural to assume that the future is completely predictable, at least hypothetically, based on the fact that even our limited knowledge of the laws of nature seems to give us a good deal of predictive power. After all, there are many things we cannot yet do, but this would seem to be poor evidence that they are logically impossible.

So in my mind, Hill wasn't trying to make a definitive case for causal determinism, only observing that it is the far more natural conclusion to draw, based on the planning-oriented way human beings interact with the world.