I agree that "morals" only exist in the abstract but that doesn't mean they don't exist. Triangles only exist in the abstract but that doesn't mean we deny their existence, or ask for evidence of their existence.
Well, yes, I agree that it's largely subjective but so are all moral precepts. That doesn't mean they aren't logical. Natural rights theory comes to a moral conclusion (which is largely subjective) but that conclusion is based upon biological and psychological axioms (which are largely objective).It's still a theory, created by man's mind. But yeah, I see where you're going with this.
So, you can disagree with the morality of the conclusions, but I don't know why anyone would. I mean, do you think that people living in accordance with their wills is immoral?