First off, I have no problem with them in the game and I do not think it promotes such things.
That said, the quoted statement may not be entirely true. What is "good" for society may not be "good" for the individual, which may result in some questionable outcomes. For example, a LG society may imprison and try to reform a clan of Orcs that has been marauding the region. Part of that process may be training them to live in a "proper" society. Forcing an imprisoned race to work (to learn the value of it) is not all that different than slavery, even if the cruelty is not there. Our own (Western) history is fraught with horrors brought on when we acting to bring civilization to the "savages".
(if it were simple, there would not be the 9 alignments of old)
Civilizing the savages is not what I had in mind when I said stopped.
I was thinking more along the lines of the dragon terrorizing villages and the PCs either slaying the dragon or driving him far away. Or going into the Underdark to rescue innocent captives who were taken to be sacrificed to Lloth.
Though stopping Orcs from raiding homesteads by teaching them to farm I don't see as taking their freedom away or making them no more than slaves.