It's not a problem with the tanking class
It's not a problem with the game, it's a problem with the players. There are actually parts of the game where the developers have explicitly allowed players to have an easy time clearing trash by just knocking them off the edge. False Emperor is *famous* for this, such that there is actually an area in that that it's best to explicitly tell players to *not* knock enemies off the edge because there is grating in the KB area that often causes enemies to bug out and/or reset.
Knockback is a utility function. Just because some players are idiots and don't understand that it's a utility function rather than their AoE damage ability (I've met so many bad Shadows that think that Force Wave is their AoE damage ability and it just makes me facepalm because Force Wave does such marginal damage while scattering my tightly packed mob) doesn't meant that it's a problem with the game. Making all enemies in FPs immune to KB would just make KB only really useful for PvP, which is just boring; I *like* being able to throw my opponents off of the side of ledges and, when necessary, pushing them around so that they stack up better, not to mention that, from a mechanical standpoint, pulls like Harpoon and Force Pull are identical to pushes (it's just a push in the inverse direction with a cap on the push distance equal to the current distance between the targets).