Pretty sure it's related to how Bungie has their shooters work.
Because of the 1st person aspect of the game, developers typically have this delimma regularly.
Do they have bullets come from the gun like they should? Or do bullets come from the camera, like the player sees?
Basically, if you fire a bullet, it comes out of your forehead. I remember Halo 3 did this and it always looked funny because you could see it so clearly in the theatre mode.
Of course, what this means is that in the game, if you can see it, you can shoot it. Doesn't matter if you gun is actually being obstructed by anything.
To test this go stand next to a wall on your right side. You'll notice that even though your gun has a wall in front of it, you can still shoot around the corner because you can SEE around the corner.
ADS does a good job of hiding this, but it was much more obviously in Bungie's Halo games.