I don't think the problem people seem to be having is that it's wrong to do, but more along the lines of, okay here's a quote from something I read in researching this "In particular, the show depicts people's attitudes towards the Dolls as objects or pets rather than as human beings. Reactions to the show from viewers and critics focus on this aspect, inviting a reading of Whedon, the writers, and the camera as being exploitive, rather than the show being a story of exploitation." I'm not saying I agree with that, just that that's what these people seem to be concerned about more so than the idea of it being the wrong thing to do. The idea of how the subject matter is dealt with rather than what it is.
Ironically, the attitude of her handlers toward the dolls is quite similar to the Watchers' attitudes toward the Slayers, and almost certainly the Potentials as well. Both Slayers and Dolls were views as resources of one sort or another. Slavery? We're led to believe that the Potentials are placed in the hands of their Watchers at a quite early age. Kendra actually said that her parents were paid for her. And no slave owner had authority over life and death any more that the Watchers and their Slayers. In fact, the Watchers literally worked their charges to death. Prostitution? That's a subject that never came up on "Buffy," but almost certainly over the centuries, some of the young girls were sexually abused. Probably the attitude of "Why not? She's not going to live long enough to do anything about it anyhow (and don't delude yourself into believing that a Slayer's strength could always overcome the intimidation that she had been feeling for years as a Potential).
Even the characters and conditions on the two shows could be seen as parallels:
(Indulge me here, I AM making a point)
The Dollhouse/Watchers - The the "management," controllers of the Dolls' living conditions, training, and assignments.
Alpha/Angel/Angelus - Dark, sinister lover from Echo's past.
Boyd Langdon/Giles - The authority figure who sees his charge as more than just a resource, who actually cares about and tries to protect her within the limits that his superiors dictate.
Topher/Willow - the resident genius with a less than assertive personality.
And, of course,
Paul Ballard/Spike - with whom Echo, during the course of the series, goes from adversary to intimate friend, if not lover, who dies at the conclusion of the series, leaving Echo regretting that she had never been able to make him aware of her love until it was too late.
Even the ending of the two shows, with both Sunnydale and the doll house destroyed.
(And here's that point)
Joss re-made "Buffy The Vampire Slayer," with a few specifics (particularly science instead of magic being the driving source of the action) changed, and called it "Dollhouse." But in the case of "Dollhouse," with its bigger scale and more obvious elements, the viewers and critics saw what he, whether consciously or not, was doing., and didn't like it.