NG, NE, LN, and CN actually are able to be protected from via a cast spell. It's only the rare TN that isn't. It's true you can't double cast to get +4 vs non mind-effecting spells from them though but if that does actually stack it might be an oversight by itself. I don't personally think something like Neutral Evil is an overpowered alignment, but if a protection from neutral is made castable it ought to be for true neutral only. One spell for five alignments -four of which are already covered- is a lot.