While I agree with you, it actually makes a fair amount of sense, given that Diana is the latest scion of a society which has existed more-or-less unchanged since Greco-Roman times. (Well, except for kangas and mental radios and purple rays and whatnot, but c'mon, cut 'em some slack.) Back then, slaves were not only accepted, they were common - every empire of any size or importance had them, and they were a part of everyday living. So while we may assume that the Amazons were a TAD more enlightened than that back then, it's understandable that they wouldn't react to slavery with quite the amount of shock and horror that we would.