Like you said, we know that cannibalism causes prion disease. We know this. Thus, we are not resistant to it. So from an evolutionary perspective you could make a strong argument that what actually happened was that those humans who didn't engage in cannibalism and die of prion disease were heavily selected for. Which is probably why you don't see many humans eating the dead. Now that last bit is speculation, but all this is speculation. But based on my own introspection, I'm imagining myself stumbling upon a recently dead human corpse, I'm almost certain my innate reaction -- (well, almost surely innate, and not socialized, since I've never been explicitly taught how to react in such a situation) -- is that I'm definitely not going to fucking eat that corpse. In fact, the only humans that do seem to still engage in cannibalism do it out of some social ritual.