But why does who willed it to happen actually matter? What is the POINT of evaluating a moral act, of determining whether or not something is ethical?
Don't forget we're moving away from "how can I avoid being hit by rocks" as a criterion. If that's all it was, you're right, it wouldn't matter how it happened other than you might need a different strategy in dealing with people-nature versus cliff-nature. But the moral claim being made is that you need to do the evaluation
in full, which includes not only some vague prohibition about rocks but includes one's state of mind, goals in life,
view of life, views toward the betterment of others, and so forth; all of this goes into whether or not you should shove rocks around. In fact this divides into whether you
may do so, and whether you
should do so, which is another part of it. Per the famous dictum that an "is" cannot yield an "ought", the moral framework at minimum should outline what you
ought not to do. It becomes more involved to find out what you
ought to do, but for this part (which is in a sense elective) you will definitely need the moral will of others to be in your mind because otherwise you won't be able to help them go through this same discernment process. You might very well be able to help someone else do it far better than you can bootstrap yourself to do, and hopefully someone will come along to help you too. There are people, I think, who seem to be of the mind that you should just mind your own business and not try to help people. I think this is a bad mindset, but it does raise the question of what the best way is to help others, which is by no means obvious.
I want to go back to another comment you made, at the risk of derailing further:
we seem to be in agreement that it doesn't make much sense to be mad at a rock for crushing your car.
I am actually not 100% in agreement with this! It all depends on what we mean by "mind" when we assign blame to minds that will bad things upon us. I am quite open to, even fascinated by, the idea that mind and intelligence are concepts far deeper than just "organisms with brains". And I will remind you that Catholics do believe in spirits, and that they are actual beings that do things. They can even affect the material world. Now before we get worried that this is mystical voodoo, the term "beings" and "spirits" may be misunderstood as meaning ghosts and invisible stuff that flies around like Casper and Nearly Headless Nick. It's a bit trickier than that, but without really getting into it Catholic priests for example often report mysterious and irritating things happening to them as they're about to do really important deeds, like wasps stinging them in the face of out nowhere and so forth. You can believe that claim or not, but it's not out of the question for a Catholic to wonder whether literal demons fling rocks at people sometimes to try to dissuade them doing good things, to make it too much trouble, or too painful. From this standpoint even pains caused by natural elements may have been inflicted 'on purpose' to hurt you, and for specific reasons. Sometimes individuals feel like the world is out to get them, and I think there may be something to this. So yes, it's possible to actually get upset at rocks flung by nobody (not the rock, really, but the forces you can't see behind it). I mean, heck, once we allow in free will as being something not bound up in materialistic nature, a whole host of other possible things are going to come along with that, no? Once there's another domain of activity you can expect it will be active.