No, it's a charity service to those souls. You see, they get really bored sitting around for eternity with nothing interesting to do. You're doing them a service, charity work, giving otherwise idle souls a purpose in their afterlife.
This I can kinda get behind:
Necromancer: So hey guys, what's up?
Restless soul: Eh, nuffin. Bored. Just sitting here for eternity with nothing to do.
Necromancer: Well.. wanna go kill some people? It'll be fun!
Restless soul: I don't know.. sounds like a lot of work. Is it far?
Necromancer: No, no, it's right over there. *points to human army coming down a nearby hill*
Restless soul: Eh, something to do I guess. Come on guys, get off your lazy asses and let's go kill some dudes!
Army of restless souls: *roar*