I see this as a good thing. It makes hitting the mask off easier compared to trying to get a non-blunt object to hit the back of the head. It also puts the Toa and the villains on equal footing. The villains have weapons specifically designed to catch and then rip masks off of our beloved heroes, rather than clumsily trying to get enough force in your sword swing to hit the eye stalk successfully. It only makes sense that there be a similar advantage the Toa can use against the skull villains. Every hero and villain needs a weakness. People act like that's a bad thing, when in all reality it's a very good thing when it comes to playability and imagination. Honestly, my biggest complaint is it's still unintuitive; knocking the mask off with the horns requires an upwards swipe, and all of the Toa's weapons are designed with a downward swipe in mind. Well, depending on how the weapon is wielded, the weapon can perform any swipe effectively.