is more because there aren't any other (known to be) sapient species in real life. Most fantasy settings that contain a larger array of sapient species define cannibalism more liberally. Usually it's used to refer to eating another sapient, or sometimes humanoid, being. For example, in Pathfinder it is still considered cannibalism (and an evil act that will get you promptly expelled from, say, the Pathfinder Society) if an orc eats an elf or human. Similarly, in World of Warcraft, the Forsaken's Cannibalize ability works on any humanoid or undead corpse, despite the fact that Forsaken could technically be said to be more closely related to an Azeroth-native primate than to a draenei.

Mary Sues, on the other hand, seem to be permitted under the argument that, given how poorly-written they are, they aren't fully sapient. Eating them isn't cannibalism for the same reason that killing them isn't murder. While that's the official PPC stance on the issue, however, I wouldn't be surprised if more than a few agents disagree, especially those who don't go out in the field. It's one thing to know on an intellectual level that a Sue isn't really "human" - it's another thing to know it on an emotional level. You're still eating something that looks like a person, and that's uncomfortably close to eating a person for most tastes.