Well heck, I was hoping one of you people would show me the way here. 
I don’t have a position, but here’s what I think.
The theories of causation I know about are the counterfactual account, the manipulationist account, and this other account I forget the name of. As far as I know, none of these accounts would make the relationship between my actions and Steve’s situation a causal one.
The counterfactual account tries to analyze causation in terms of certain counterfactuals. But in the case I’ve given, there are no counterfactuals that hold about Steve’s situation that turn on the question of what I do with the box.* If I had decided to put a different number in the box than I actually did decide, Steve still would have ended up in exactly the same situation.
The manipulationist account says there is a causal relation between A and B just when you can manipulate B by manipulating A. But the thing to say here is similar to the thing to say about the counterfactual account–you can’t manipulate Steve’s winnings by manipulating the number of stones in the box.*
Meanwhile, the other account I can’t remember the name of involves the question whether there is information that is somehow preserved and transmitted from A to B. This is not happening in the case I outlined.
So as far as I can tell, there is not a causal relation here, at least not on any account of causation I’ve heard of. Yet it is very plausible to think that one can not deliberate about things that are not within one’s power, and I don’t know how “power” could be explicated in any but causal terms.
So as far as I can tell, it is not within my power to say how much Steve is going to win, and this should mean I don’t think it possible to deliberate about whether and how much I’m going to help Steve win. Yet I do think it possible–in fact, I find it impossible (for me) not to think I am in a position to deliberate over what to do about Steve’s situation.
Perhaps a new account of causation is needed. Or perhaps instead what is needed is a different account as to what can be deliberated over. Or maybe I need to find a way to divest myself of the unshakable feeling that I have some responsibility concerning Steve’s situation. Or finally, maybe I need to fight the hypothetical and deny that there could be such a situation as the one I’ve described in the OP.
So basically, what I think is, WTF mate?
-FrL-
*Both asterisks above point here. It might be argued that there are counterfactuals that hold concerning Steve’s situation that turn on the question of what I do with the box. Since all I know is that the number of stones in the box is the same as the number of millions Steve wins, I am in a position to say something like “I now intend to put five stones in the box, but were I to put 10 stones instead, Steve would win 10 million, and were I to put no stones, Steve would win nothing” and so on. This looks like a set of counterfactuals. But they don’t seem to be the kind that are supposed to support the existence of a causal relation. For these are, as it were, epistemological counterfactuals rather than metaphysical ones. What’s that distinction? Well, I’m making this up as I go, but: An epistemological counterfactual is one whose truth or falsity turns on what the person making the counterfactual judgment knows or doesn’t know about the situation. A metaphysical counterfactual is one whose truth or falsity turns just on what the objects are that the judgment is about, and what their actual relationships are. So, for example, my judgment “If I were to put 10 stones in instead of the five I’m going to, Steve would win 10 million instead of 5 million” is an epistemological counterfactual because it is only true by virtue of the fact that I don’t know how many stones I am going to put in. Meanwhile, the counterfactual “If I had put 10 stones in the box, Steve would have won 10 million” is a metaphysical counterfactual because it is false by virtue of facts about the stones and the lottery and Steve (etc) and their relations only. Anyway, it seems like counterfactual (and manipulationist) theories of causation want to talk about a metaphysical relation, not an epistemological one.
That was hardly clear at all, and I’m hardly clear about what I was trying to say, but I’ll leave it up there in case anyone wants to chew on it and comment.