Because the writer says it is
In real world history, necromancy was limited to speaking with the dead and Christianity claimed that mediums were actually speaking to demons. No magicians ever claimed to animate corpses for labor until pulp writers invented the concept in the 1930s. Before then, the dead came back to life purely under their own power as ghosts, vampires, ghouls, werewolves and other man-eating monsters.
For an example of how arbitrary the morality is, look no further than the Necroscope series. The protagonist is a medium or "necroscope" who speaks with the dead and may call upon them to rise from their graves and assist him. The antagonist includes "necromancers" who mutilate corpses to learn their knowledge, which the dead souls feel despite being dead. Otherwise the dead don't care about the condition of their unfeeling remains.