Can't Hollywood give the roles of Native Americans to Native american Actors?

So today, I saw an article about Johnny Depp's new movie role. He's going to be playing a role in the movie The Lone Ranger. What role is he going to be playing? Could he be the Lone Ranger? Could he be playing the bad guy? No, he is playing the Ranger's faithful companion, Tonto.

Sure, Depp is a great actor, but why did they put him in that particular role. Couldn't the casting director find a Native American actor to play the role? Did they have such a huge need for a famous star to be in the movie? I'm not familiar with Armie Hammer. He's been in a few movies that I haven't watched, but he's no Johnny Depp.

Depp could have had the title role and a Native American could have played Tonto. Just saying.