If morality is objective then we have this:
It is objectively true that people should not kill one another.
If morality is subjective, then we have:
I prefer that people not kill each other.
There is surely a huge semantic divide between these variations: but it’s not clear why it will have any behavioral force at all. Why does a want justify an action less than an objective imperative?
Indeed we could rewrite all of knowledge in either subjective or objective format, as we wished. Take, for instance, my belief in the tree in front of me. If my senses track objective truth, then we have: it is objectively true that there’s a tree in front of me. If, to the contrary, reality’s some sort of subjective construct, then we have: I prefer to believe there’s a tree in front of me.
Now, if I believe the tree’s there, I’ll walk around it. If I simply prefer to believe a tree's there, I’ll do the exact same thing.
Where's the relevance of the ontology?
I'm not sure at the moment, but I think the fact that we do respond differently to things we deem truths and things we deem preferences lends support to Nozick's claim that truth has an inherent value--one that must be factored in to any search for the good.