The indians did shoot instinctive because they knew that there life depended on them being able to shoot standing up lying dow on a hores when you are in battle and in many situations where you must shoot fast and you don't have time to aim. They had the knoledge to know that if they relied on thier natural hand eye cordination that they who be using the most reliable thing of all your brain which is better than the most enhanced computer ther is.
And the last time you spoke with an Indian who has actually shot a bow in battle and from horseback was when? Even the Indians I grew up with were three generations after the great Indian wars. None of those great-grandfathers who actually used bows in battle were left to talk about it.
edit- that sounded a little too short and argumentative, so I thought I'd better explain further...
Point is, you can't KNOW that Indians didn't use an aiming system. You can only guess. I can't KNOW that the way I was taught to shoot is truly the way they did it. I know the Comanche and Apache kids I grew up with were taught the same way I was. To pull the arrow back close to the face and eyeball down the arrow shaft. Some of us shot with a split 2-finger release and some shot with the pinch draw.
Even then, when you make your living with a bow in your hand, I'm sure shooting becomes a lot more automatic than it ever will be for any modern man but I seriously doubt the indians were totally committed to instinctive shooting. I think there's a lot of modern white romanticism going on when someone says indians were purely instinctive shooters.