Maybe it's to do with the fact that dreams are a result of the more ancient brain sorting itself out (i.e. moving memories to long-term and trying to understand the days' events etc.). Remember that cats and even rats dream, so maybe the high level logic we see as missing is not meant to be there; it's like we have the additional conscious layer on top that we are aware of, but that other animals do not have.
Maybe we are unique in the animal kingdom of being able to LD - (or perhaps chimps and gorillas can? I don't know).

There's another interesting facet to the "logic not working" and that is, why are normal dreams so vague and dreamy? That I guess works with Sageous' memory explanation, but it could also be because dreams don't normally have a close link with the conscious perception centres of the brain.