I'm surprised that you say that Native America isn't being taught as diverse, kherbert! I was lucky enough to go to a school (East Coast) when I was a kid that spent a lot of time learning about some of the different NA cultures. We built wigwams and teepees and pueblos and longhouses and totem poles, and learned about all the different ways that they lived. It was one of our favorite things to study in school.
It's too bad to hear that not all schools do that!
I do agree that a lot of schools spend many, many years studying U.S. History, and not so much studying the world. A lot of schools spend about 8 years studying U.S. History and approximately 4 years studying world history. Our homeschool curriculum reverses that and does 8 years of world history/geography/culture and 4 years of U.S. history. They also have you start with world history first, so that you understand U.S. history in the context of the history of the world, and not as an isolated thing.
A lot of kids basically know almost nothing about history prior to Columbus. When I was in school, we learned about world cultures some, but I don't think it was until AP Modern European history (so 10th grade) that we really learned world history in detail, and then it was an awful lot of history crammed into one year!
I think there is value in studying history from the perspective of something specific, though. Texas or other state history, something like that. When I was in college, I took a semester of Jewish history, and it was really fascinating and valuable to see history from the perspective of a particular people. It let you see how things fit together in a different way, without being overwhelmed by covering what was going on in so many diverse places. At the same time, while I think a semester or year of state history is good, more than that might be a bit overkill!