"The Chiefs Daughter" by Rosemary Sutcliff

While reading the "The Chiefs Daughter" by Rosemary Sutcliff I came across a very surprising event. In this selection, a young Indian girl meets an Irish prisoner and when a stream that brings many important resources to the tribe, begins to dry up, their relationship is put on the line.
I didn't understand why, if the boy was a prisoner of war, did Nessan's tribe allow him to wonder all over without anybody accompanying him. It sounds to me as if he is an Irish boy living a young Sioux boy's life, which is a lot better than being locked away in a small, dark jail cell that you and I pictu…