This decision was really important, and its importance is missed by a lot of people who don't understand the implications of NAGPRA (Native American Graves Protection and Repatriation Act).
The act's wording was, in some people's opinion, sloppy.
It was done in response to the fact that Native Americans thought the remains of their ancestors were not being accorded appropriate respect. In many cases they were right. In one example, a highway crew came across a 'pioneer era' cemetary. The bodies of the whites were reburied in another cemetary. The bodies of a Native American woman and her child were sent somewhere for study.
Anyway, there was a big political push to strictly regulate the handling of physical remains and grave goods (stuff they were buried with) of native americans. (Side note: archaeologists in general are not as interested in human remains as in the stuff we leave behind). It passed.
What all this has to do with Kennewick man, is that, by legal definition, K.M, being the remains of a person who lived here before the whites arrived, is Native American...even if he is in no way related to the tribes that are here now. Thus, the tribes (who wanted him reburied without being studied) might have had authority over his remains...even though he wasn't one of their ancestors.
This case had huge potential implications for physical and bioarchaeological research in the U.S. We could have lost the right to study the people who were here before the current Native American tribes.