A lot of that is thanks to the immersion of the game. I've picked this game up recently and it has been one of the only games in a long time to really grab my attention for more than an hour or two. You can even sometimes bug into this pose in first-person when switching from third-person, thus I can only assume it's already in the game as such. The animation is in the game as evidenced by walking up too close to any object, letting you know you cannot aim. In first-person however, the weapon is always up and aiming. Currently in third-person, your character will lower their equipped weapon after a few seconds as if not currently in use.