Is there a reason why Zilean's bombs cannot hurt towers?

Why not Zilean? Is it because the cooldown is too fast?
As if that was even an obstacle. If you can give Master Yi bonus damage on his Alpha Strike towards minions and monsters then you can debuff Zilean's Time Bomb damage if used on towers. Kinda unnerving to see Ziggs and Tristiana popping charges/bombs on the towers and feeling inadequate knowing you can't do the same. It just seems unfair. This is preferable to nothing but an incredibly slow auto that does jack all to towers early game. And its not like you guys haven't had a ton of practice giving various other champions with non-bomb related abilities/buffs that can hurt towers.