Kassadin's W doesn't proc on turrets

not all aa modifiers do

it hasn't for a very long time so im pretty sure its intentional and like the previous poster said not all aa modifiers do (ex. jax w passive and i think the w itself i don't remember for sure thou) also not all abilities that apply on hit effects can hit towers (ex. ezreal q)

The only time it applied was back when Armor Pen worked on towers. And when Kass' W gave 50 Armor Pen.