The developers have already told us that there are no plans for slave trade at the moment, however, I would like to continue this discussion to show how the points you made are flawed.
The media seems to be ok with games like Grand Theft Auto and Saints Row right?
Bob also plays a game called Grand Theft Auto 4, where he plays as a veteran of a (presumably Yugoslavian) army that committed war crimes and atrocities during the duration of the conflict. Bob's character now is involved in multiple crime rings in a fictional New York city, where he kills rival gangs, cops, and random civilians at will. Bob also plays a game called Mafia II, where he plays as Vito Scaletta who makes a living working for mobsters killing anyone and everyone he is asked to kill for money. With this money, Bob buys cars clothes, and more weapons to kill even more people.
Virtual slavery isn't any worse than virtual murder. It's just that casual gamers have become "hardened" to games like this. Slavery in a game is just so "edgy" that most people are turned off to the idea for historical value and authenticity.
On second thought, you are right. I kind of feel bad for the virtual slaves....