I agree that scientific theories make no moral statements.
If it was somehow possible that they could, and they did, I suppose we would make moral judgments about them, which is a silly idea--- "the laws of conservation are EVIL!"
However, that is different than saying that scientific theories have no moral implications.
As you said, scientific theories are about the way things are--- what things are made of, the way things happen.
Well, the way things are has implications for the way things ought to be.
This holds even if you subscribe to the is-ought distinction of Hume.
Kant explained why: while Hume may be correct that "is" never implies "ought", "ought" always implies "can".
So if human nature is as not elastic as Marx thought it to be--- which I think we can agree turns on a question of science, not morality--- then the morality of his system is rendered false, because it demands man ought to do what he cannot do.
In other words, the (scientific)fact that human nature is far more stable than Marx envisioned has the moral implication that Marx's system is immoral--- but only if you accept as a further premise that a politcal system which demands man ought to do what he cannot do is wrong.
In other words,
1. A political system that demands man ought to do what he cannot do is morally wrong. (moral fact)
2. (Scientific discovery)Man's nature is not elastic; therefore it is not possible that his nature be changed in the way Marx intends.
3.(Conclusion)Marx's theory is morally wrong.
Similarly, the discovery about curare's affect on the patients has moral implications if we assume as a further premise that it is wrong to torture patients.
I'll admit that, unless one admits that there are such things as moral facts (such as (1)) then one does not have to admit that scientific theories can have moral implications.
But in that case, nothing at all has moral implications.