Ashe's Q 1st passive stack isn't triggered on turrets, but stacks are maintained when hiting turret

You won't get the stacks of Ranger's Focus if you 1st hit on turrets, but you will maintain the stacks you already had when hitting a turret. Also you can use the active on turrets as well. Is this intended? I'm so confused. Shouldn't be "does both" or "does none"?