Ask yourself, "How does my pirate ship attack pirates during a defense?" ... the logical answer is with cannons! However, LP's that buff cannons don't effect the ship's attack. A minor oversight, but it would be in theme with pirates, cannons, and LP buff descriptions.
Gun towers, ground pounders, and mortars are also cannons by your logic. Heck, bunkers, gunners, and HGs are cannons too, just at a smaller scale.
