I'm sure there is more than one aspiring native american actress in the entire world... It doesn't have to be Q'orianka Kilcher. Also to me, Tiger Lily doesn't seem like a big role, despite its placement in the credits.
Having Rooney play Tiger Lily is the exact opposite of diversity. Hollywood is inundated with white people. How does casting a white female, in the place of a native american "diversify" an already white washed industry! Native americans need representation. The fact that you can only name one native american actress off the top of your head speaks volumes. You know what would have been seen as "progressive" (although it shames me to think)? Sadly enough, in this day and age, it would have been "progressive" to have a native american actress play an actual native american (shock horror). People of colour need representation, women of colour need representation. Why should we white wash the few roles that should be impossible to white wash?
Also just because an actor/actress plays a role that does not reflect their own ethnicity, does not mean it equates their acting to "great" or "Oscar-worthy". I bet there are many unknown, talented actresses that could do a great job. Quite frankly I'm not impressed by Rooney's performance in the trailer.