Why was the "No Darkside Ghosts" rule broken with the Nightsisters?

Question

Originally, the Expanded Universe featured some Dark Side ghosts. However, this story element seems to have been eventually retconned away.

This is stated pretty explicitly in Star Wars: The Clone Wars.

Force Priestess: Do you come to us with only good intention and light in your heart?

Yoda: I do.

Force Priestess: Then the blessing I give, and the training will begin.

(Destiny)

Narrator: All attention is focused on Yoda, who claims to hear the voice of a long-dead Jedi. Convinced that he is guided by Qui-Gon Jinn, he finds five priestesses who will teach him how to manifest consciousness after death.

(Sacrifice)

It's been made quite clear that the Bane who appeared in that episode was only an illusion of some kind, as the Sith are incapable of living beyond death.

Revan and Bane were earlier cut from an episode of The Clone Wars because Lucas didn't like the implications of Sith continuing to exist after death, which Filoni agreed with.

Despite all that, Nightsister ghosts appear in an episode of Star Wars: Rebels.

While not Sith, the Nightsisters were darkside Force users.

Imagine a planet where the female population consists of nothing but mysterious witches, powerful in esoteric dark Force powers. (StarWars.com)

While it should be noted that while the Nightsister ghosts were not as powerful as Jedi ghosts (they couldn't leave their cave), they were capable of possession, a skill apparently unknown to the Jedi, and they definitely kept their old personalities and consciences.

Rebels has the same creative team as The Clone Wars, so it's unlikely they would have forgotten or disagreed with their own rule.

So why did Filioni break his own "Darksiders can't live beyond death" rule and what repercussions does this have within the canon?

Answer 1

Short Answer: They're almost certainly not "force ghosts".

Remember that as explained in the Clone Wars episode 'Voices', a "force ghost" is someone who has died, become one with the cosmic force as all living things do, but have been able to retain their individuality and return from the netherworld. Such beings are unbound by time and space but are strongest/most able to communicate in places where the force is strongest.

Qui-Gon: I am a manifestation of the Force. A Force that consists of two parts. Living beings generate the Living Force, which in turn powers the wellspring that is the Cosmic Force.
All energy from the Living Force, from all things that have ever lived, feeds into the Cosmic Force, binding everything, and communicating to us through the midi-chlorians. Because of this, I can speak to you now.
Yoda: See the future, you can?
Qui-Gon: I exist where there is no future and no past.

Since much of it is left intentionally vague and mysterious, it's difficult to say with any authority exactly what is going on with the Nightsisters and how much of their abilities are an alternate expression of the dark side and how much of their "magicks" are something else. In 'The Disappeared, Part II' Mother Talzin clearly states that she is not a natural force user. Her powers are to some extent derived from some other source or method.

If I had to theorise what those apparitions in the cave might be, I'd suggest that they're some kind of living imprint, recorded and generated by that altar which is itself some kind of unknown Clark's Law technology (may even be somewhat similar to holocron technology.) Hence it's ability to temporarily imprint on living brains, as well as the limited range and that destroying the altar seemed to also eliminate them.

It could be that this is an attempt to circumvent death by trapping a consciousness in an unnatural way. The Sith are said to be similarly obsessed with escaping death, that they don't believe in the greater cosmic force and are entranced by the physical realm. Now the Nightsisters aren't Sith, but this may be a common belief of dark side users who are more inclined towards a selfish outlook.

As seen in 'Massacre' the Nightsisters have demonstrated similar feats of necromancy before, though in that case it was in animating their own mummified dead as cannon fodder. Not quite the same situation, but it does still involve possessing bodies (unoccupied ones in this case) and the creepy green "magick".

Also in 'The Disappeared' we see what amounts to "magickal" artefacts that can pull the "spirit" out of living beings and store them in a "living orb". If something similar happened to those two Nightsisters in the cave then although their physical forms are gone, they're not actually dead but trapped in that place, unable to truly live or die (at least until Ezra destroys the altar.)

Answer 2

Although it's not the same canon, it might be instructive to look at the Sith ghosts in The Old Republic. The distinction is that while Jedi pass on to afterlife and then can manifest, for the Sith it's all about avoiding that very final moment of death. Basically, a Sith ghost is (in the words of The Princess Bride) only mostly dead. Although their body is dead, their spirit can live on because of its connection to something; usually a location, although Inquisitors in that game learn to bind ghosts to themselves instead. In theory, they can recover from that state given sufficient power, though Vitiate is the only one ever shown doing so; mostly they just linger until finally whatever talisman is keeping them from the brink of death is destroyed and they finally pass on. (Exar Kun infamously survived over 4000 years in Legends in the temples of Yavin IV before his last connection to the living realm was blown up.)

Like I said, this isn't Canon canon, but I would imagine that the writers' thought process is similar: that the Nightsisters weren't quite dead, and that once they were it would be 100% final for sure.