Well, the way I've always seen it is when a creature dies and goes on to another plane, they aren't infused with negative energy all that often. I think creatures that pass on are more like outsiders, more like petitioners. And if that isn't enough, I think them being in the positive energy plane so long that they begin to absorb it's essence and become part of it means they'll be an outsider attached to the Positive Energy Plane in the same sense an Archon is to Celestia.
And thank you very much.