As I read the documents, I learned why. It seems that during the Vietnam War special forces in the war department had sent undercover experts to comb American Indian Reservations looking for talented scouts, for tough young men trained to move stealthily through rough terrain. They were especially looking for men with outstanding, almost supernatural, tracking abilities. Before being approached, these carefully selected men were extensively documented as experts in tracking and survival.
With the usual enticements, the well proven smooth phrases used to enroll new recruits, some of these Indian trackers were then enlisted. Once enlisted, an amazing thing happened. Whatever talents and skills they had possessed on the reservation seemed to mysteriously disappear, as recruit after recruit failed to perform as expected in the field.
Serious causalities and failures of performance led the government to contract expensive testing of these recruits, and this is what was found.
When questioned about their failure to perform as expected, the older recruits replied consistently that when they received their required military haircuts, they could no longer 'sense' the enemy, they could no longer access a 'sixth sense', their 'intuition' no longer was reliable, they couldn't 'read' subtle signs as well or access subtle extrasensory information.
So the testing institute recruited more Indian trackers, let them keep their long hair, and tested them in multiple areas. Then they would pair two men together who had received the same scores on all the tests. They would let one man in the pair keep his hair long, and gave the other man a military haircut. Then the two men retook the tests.
Time after time the man with long hair kept making high scores. Time after time, the man with the short hair failed the tests in which he had previously scored high scores.
http://www.sott.net/article/234783-The-Truth-About-Hair-and-Why-Indians-Would-Keep-Their-Hair-Long