I was told that some tribes did not cut their hair as it was natural (as you stated) and that natural should be left to their natural state. Well, that's what I was told. There was a lot more to it but I can't really remember. There are other reasons also, one was that was how some of them felt about their hair, others that was what guides told them. Sometimes it was kept long for practical reasons. I wish I could remember it and it really bugs me that I can't.
But anyway that was what I have been told or I have learned. But personally I like my hair long. It suits me =)