I mean yeah. But Derika is a more militarist society. It takes pride in it's military, for it to use another nations army to protect itself seems kind of cowardly. Not to mention, the logistics of a slave soldier. They cost money to feed, heal, house and search for when or if they escape. I get your point that the US does it all the time, but Derika isn't a democracy (ish) it's an empire.