The buildings, walls, sculpture, paintings etc. of the Vatican are considered its national treasure and on the list of World Heritage Sites. The walls were built around 800 AD and don't surround the city completely. The Pope himself can't unilaterally tear down walls or dispose of everything in the Vatican according to whim, anymore than a U.S. president could sell off our national treasures. Also, anybody can go through the Vatican walls at anytime. There are no border guards asking for papers from those who enter.
Every wall in the world can't be used as ammo just because it's an ongoing issue with Americans.