I don't know how it technically works in-lore for the Forgotten Realms, but I can believe ghosts being capable of any alignment while vampires, liches, more for lack of a better term, physical undead are inherently evil. After all, ghosts are typically remnants or echoes of a person's soul, so it would make sense that they share an alignment. If I were running a game I would say that most undead are inherently evil aligned unless there's some kind of magical or divine interference. Meanwhile ghosts are stritcly limited to acting within the alignment they had in life and are unable to act outside of it, sort of enforcing the idea that they aren't a whole person, and have lost the depth that life gave them.
Yes ghosts and undead are basically opposites, Ghosts are the souls and undead are the soulless bodies powered by negative energy.
