You assume that these people have no 'law' of their own. Though I cannot say either, it is entirely possible that distinguish between the ways a person is killed and they are punished accordingly. Simply because it is not your law doesn't mean they don't have their own.
The article states these tribes are rivals. They may consider themselves at war. Soldiers are not tried for killing other soldiers (within reason) in wartime, so why should rival tribesmen be tried for killing other tribesmen when they are at war?