Economists do stop short of articulating a fledged theory of human nature.

No, they simply don't overtly state their theory of human behavior. The theory is there and evident by the claims and structure of economic theory.

However, they elucidate human nature by studying how people actually interact with one another.

Economics is pretty weak on empirical research.

Any student of human nature would be remiss to pass over this profession.

Oh, I am by no means suggesting that economics has nothing of value to offer. It can be quite illuminating provided an economist isn't the one using it.

You seem to be assuming that they have some type of 'theory' and then try to 'fit' human actions into that theory.

Nope, I am observing that economics is founded on a set of specific philosophical premises and a specific cultural framework - and from that perspective they selectively construct a view of economic activity which is neither all encompassing nor objective.

This is always a danger (even in natural sciences), but the general idea is that, when done properly, the evidence suggests the best theory to us.

Right, and as I stated in my original post, most people in the humanities and social sciences think that economists don't do it right.

If you want to effect the world in an ethical way

Academia is not about redesigning the world. It is about expanding our understanding of existence. While the results of our work can be used to to change the world, a field which sets out to change the world in a particular way is by definition not objective.

The why? (i.e., why is this the right way, what is the right way) and the how? (i.e., how do we implement the right way).

Economics does not address the former and it is not suited for the latter.

I think the first question is of higher dignity, but if you actually want to implement a moral system- you need to know economics.

A cute value statement which has zero bearing on this discussion.