They feared more mistreatment
Native aborigines view the European colonization of Australia as a belligerant action because they saw the settlement of europeans as an invasion of their land and an attempt to change their lives. There were many complaints about sexual abuse of native australian women by some europeans. Tensions grew after several massacres of aborigins.
Basically, a citizen of the United States is any person who is in possession of the citizenship, which can be acquired by being born on the territory of the US or by being naturalized. The citizens, unlike other legal residents, can choose their representatives in elections.