Why not? Myth morality is not the same as modern morality, and the situation here is not exactly analoguous to any real world one. These are brutal monsters who regularly eat humans and dwarves.
But the dwarves are brutal monsters who enslave and eat them.
Honestly, I would rather not involve ourselves in their civil war too much.
Plus, they serve the Dark, and we serve the Light!!! Therefore, whatever we do to them is completely moral and justified.
Is that why we are trying our best not to show ourselves in front of the Imperial authorities and specifically Alric?
Let's be honest, we are not the shining beacons of Light, even if you squint really hard.
How is this any different from breeding or trading horses? Horses are, after all, sentient.
Horses can't talk. That's pretty much all there is to it. Ghols and mauls and whatnot are sentient on the human level. The rest is sophistry.
Remember, the ghols stole our money, and we're claiming it back with required service.
I don't think someone stealing our money warrants selling them into slavery.
But, hey. It's not like we don't know where you stand on the matters of morality. So feel free to pitch the morally justified ways.