I'm not saying it was good or wise for them to make the "tomahawk chop" gesture while yelling in a way that the news report characterizes as an "Indian war whoop." And Scott Brown has said he doesn't condone that sort of acting out, but I just want to specify that the staffers' gesturing and noise-making doesn't count as antagonism toward Native Americans.
Someone doing the "tomahawk chop" is himself playing the role of Indian. This Indian character making a stereotypical gesture can't be read as expressing hostility toward Indians. The Indian is his hero. At a certain level of political correctness, the tomahawk chop is considered offensive to Native Americans, but somehow it's not offensive enough to have stopped Atlanta Braves fans from doing it.
Anyway, these fake Indians, the staffers, are pretending to be real Indians, miming an attack on Elizabeth Warren on the ground that she's a fake Indian. There's a lot of fakery in there, but no one is expressing the view that it's bad to be Indian. That's all I wanted to say, and I do understand how real Indians might prefer not to be represented as stereotypical characters. And maybe they'd object even more if a non-Indian got a great job from an employer who was practicing affirmative action in hiring or self-promoting by claiming diversity.