Because exploitation is contrary to fundamental human nature. And it won't
> do to say that it can't be contrary because human beings do it (exploit one
> another).
Well, why the hell not?
> Certain things people do are consonant with what is fundamental about human
> beings, and certain things aren't. Our notions about human nature are also
> the basis for ethics (assuming we don't believe that morality is ordained by
> God).
You leave us a choice between "ordained by God" and "ordained by Jim." I don't know where to begin dissecting the assumptions packaged in the phrase "[o]ur notions." Were any of history's bloody dictators and conquerers less "human" than us? I see no reason to believe so. And certainly they all had their own notions about what is "fundamental" to existence.
-- "Hige sceal þe heardra, heorte þe cenre, mod sceal þe mare, þe ure mægen lytlað."