Playing AD kennen is like petting a cats butt against the grain.
For the majority of lols lifetime, Kayle was a support champion, playing her AD would get you raged at.
She can be played AD now, but her kit and balance basically just make her a RADC, the ability to go from ranged to melee has no consequence. When her E spell is down she is ineffective. They could quite easily remove the skill altogether and just make her ranged with next to zero change in gameplay.
Also, Riot intended Kennen to be AD or AP. They even mentioned it when people started playing him like that occasionally. AP the much more powerful, but AD the occasionally viable alternative.