The theoretical reason why she's allowed to do this while nidalee isn't, is because she takes a lot of time to get to this point, she isn't hurling out w's at any decent rate until she gets w evolve which is usually 2-3 items on a hybrid build and 1.5+ items on the ap build, and even then, just getting the evolve doesn't do too much until she stacks more ap/penetration/haste.
While nidalee can chuck spears at you for considerable damage even at level 1.
Additionally, Nidalee has a self-heal and a 3-second cooldown zero-cost leap that clears walls.
Kai'Sa can walk quickly for about one second. Similar to Zoe: Zoe doesn't have a way out when you catch her.