Because it's intentionally distinct from imprisonment to punish and/or rehabilitate. He already served his punishment, but because he has shown to be a danger t

Because it's intentionally distinct from imprisonment to punish and/or rehabilitate. He already served his punishment, but because he has shown to be a danger to society, he is removed from it until he is no longer a danger (which may be only when he's dead or too frail to commit more violent crimes).