I'd disagree. You're using the meaning in a different way. However, if it makes you think of native people tribes, feel free to suggest a better word, since, if you do think of it that way, I can certainly see why you'd think it a poor choice of words.. Hrrm, sect-ism? I mean, in general use it could refer to the lost tribes of Israel from the Old Testament. To me, it doesn't conjure up that image.
However, I'll fully agree that, especially if you are a native American, I might not see why it bothers you. For example, my nieces, when they were kids, had some silly chant they were doing that included the words, Chinese Japanese (I think it rhymed with knees, but we're talking 25 odd years ago). Afterwards, my wife, who is Japanese told me she found it offensive. I hadn't felt there was anything offensive about it--point being, that is is easy to miss why one group might be offended by something when someone who is not a member of that group thinks the offended party is overly sensitive. As I was always intimidated by my older brother, I didn't speak to the the niece's parents about it. I did what every grown, mature man does and called my mommy. I don't know what actually ensued after that but the older niece, who was probably 8 or 9 at the time, called my wife a day or so later to apologize.
Anyway, point is, I don't know if you're a native American, and even if you're not, I am sorry if you found the word offensive and freely admit that many of us can be offensive without intending to have done so.