It was sort of a lose-lose proposition for them, wasn't it? The answer to this, I believe, lies in a variety of factors I'll try to address in my limited space below:
1) Cultural differences between tribes - Some were more warlike to begin with or felt more threatened, while others had always dealt with conflict in a more peaceful manner
2) Time Frame of Contact - Te earliest tribes had little reason to fear the Europeans, or at least, they didn't know to fear them. The early explorers were small in number and the early settlements barely survived the winters on the east coast. Later contact, say with the Sioux or Cheyenne in the American West, they had already heard much about whites, smallpox, the US Army and why you shouldn't sign treaties.
3) Pragmatism - That is to say, some of the wiser chiefs were simply more practical, saw that the whites had overwhelming numbers and strength, and that to other tribes, resistance had been futile. Why not try and get as much for your people in the future as you can?Given the fact that the US government did not honor the treaties they signed, this may seem foolish, but armed resistance, in the long run, was every bit as unsuccessful if not more so.
I think that the question featured underscores the fundamental choice that all marginalized people and social orders must make. Operating under the premise that the Native Americans were a minority whose voice was relegated to the periphery of American dialogue, tribes had to make a conscious decision as to how to approach the changing state of affairs. Some chose to accommodate White expansion and preserve their tribes. This meant acquiescing a great deal to White America, but the tradeoff was that they could construct a sphere where they were left relatively alone. Other tribes felt that a stand was needed to repel White expansion as well as to reclaim what was taken away. This path of resistance usually spelled out destruction and the complete obliteration and enslavement of a tribe. In the end, such a paradigm is faced by all marginalized people that are honest to assess their situation. Either there is a sense of complicity with the majority order in order to "make do" or there is a resistance to it. Certainly, the choices do not have to be in such stark terms, especially in the modern setting where situations are more complex and fluid than they were for the Native Americans. Yet, the encroachment of Native American lands and the lack of support for tribes forced them into a static choice having to be made.