I think it's unintentional to be fair. The building assets are actually purchased from the Unreal Engine asset store, not made by the game developers. This means window bars were not added to suit the game play, but rather they were there before the game play was established. Through unfortunate coincidence, the muzzle height when shooting guns matches the location of the window bar, making shooting incredibly difficult. In real life, you would lower or raise your weapon an inch or so, but this isn't possible in PUBG.