Because they aren't unleashing the attack. They are using the lunge as a movement ability and since it doesn't contact a target it doesn't consume it. This form of attack is more like a normal charged melee as those are consumed on hit, hence why you don't lose it if you spam it. Arc upper cut, ballistic slam, handheld, throwing knives and throwing hammer all are a form of unleashing an attack. They are consumed because they can and will do damage to something so long as they are within the radius of the attack. Shoulder charge doesn't emit an attack until AFTER you hit the target, which makes it partially a movement ability and partially an attack.