Becuase under the internationally accepted rules of war (those wonderful things that keeps it from turning into out and out brigandige), you aren't supposed to shoot at religous sites, hospitals or schools.

However, as I said, the rules also say that if you shoot from those places or use them as CPs or as depots or anything else, the rules are off. Fair game. It's just like you can't shoot and wave a white flag at the same time, it doesn't work.