I was in a game as Quinn vs Sion where the enemy Sion had brought a bramble vest and was attacking my tower, I was taking damage from his bramble vest but the tower did not aggro him and kept attacking minions.
I believe because it's damage is technically counted as YOUR on hit effect, that as far as the tower was concerned, you were just punching yourself.
Whether it should do that or not (Considering ranged champs dominate melees in lane generally, if a melee can put themselves in position to attack your tower they shouldn't be dissuaded by the fact that you can auto them to force them to eat tower aggro), that is the most likely logic at play.
LOL sion must have enjoyed you killing yourself while he takes the turret
