Kassadin's W passive not working on towers once active is used

Normally when you attack towers with Kassadin, It'll apply his W's passive damage. When you turn on the W active, this is disabled, making him do less damage to towers while he is holding his active. I believe this is because his W active's damage component is specifically designed to not affect towers, but instead of being an additional component of damage ontop of the passive, it disables the passive and becomes its own bonus all together.

To replicate, load a custom game. I chose howling abyss. Put a point into W. Go up to tower when creep wave spawns. Hit the tower. Notice your damage against the tower is around 50 physical + 13 magic or whatever. Turn on your W. Damage per hit drops to only 50 physical per swing while W is active. Returns to normal when it expires or is used.

I don't know if this is intended but it seems counter-intuitive.