As far as I know there is no limit to the number of players salted earth can pull in which is why repeated pulling is so lethal in FL. You can eleminate big groups of enemies in just a few coordinated hits.
Having some kind of immunity would make it so that DRKs can still pull in an unlimited number of players but you couldn't chain salted earth of multiple dark knights at least. I'm not sure if it would completely solve the issue (I'm leaning towards an upper limit of 5 players myself) but I guess it's hard to balance the skill to ensure DRK is still a useful pick in FL without being unreasonably OP, esp. if matched against multiple DRKs.