Remind me again why Kennen's W passive AA enhancer is permanent?

Literally every other AA enhancer in the game is temporary and has a set limit before resetting itself, every onhit stack mechanic has a timer after which the stacks reset counts.

So why is Kennen an exception? Why does he get to store his W enhanced AA forever? Why not make it something that is more rewarding and less unfair? Like, from a Gameplay perspective I can somewhat understand it, but taking the essence of AA enhancement into consideration, Kennen's W passive AA enhancer is out of line.