Even when the ghouls attack you, the turret completly ignores them until ALL creeps are dead.
Every other summoned unit is focused first.
Zyra plants? 1 shot dead
Malz creeps, Tibbers, Heimer turrets, they all get focused.

This allows yorick to just spam ghouls at you under the turret and get a fuckload of free harrass in.
Imho, it should really be fixed

EDIT:
What some people don't understand here, is that yorick should still get turret aggro when he damages you, even with his ghouls.
I am talking about the following scenario:
The creep wave is pushed into your turret, yorick is just outside of tower range, you try to cs, he summons ghouls.
You now need to run circles around the turret to avoid the ghould damage or tank it, whilst the turret competly ignore the ghouls
If Yorick would walk into turret range during this he will and SHOULD get turret aggro.