This isn't morality. This is acting in the best interest of oneself. You're outlining it perfectly. The only reason tribal members act this way is because they know they won't be treated equally if they act differently (in a negative manner). They aren't doing it out of the "goodness" of their own hearts.
Objective morality is incapable of proving. It requires blind faith in what you're doing is correct. Tribal members act in the same way that dogs do (not to degrade them to animals, but this is the truth); they know that if they don't adhere to the laws/customs of the village, they will be treated poorly. Members of a tribe aren't born inherently knowing how to behave within the community. They're conditioned to behave in such a way by their elders.
So where is this "goodness" located other then in the logical faculties of the mind, and the emotions created from them. When helping people, and doing the 'right', 'correct' or morally right thing and you have a good feeling is reliant on the conditioning of that individual. So can humans be inherently good or evil? Not by definition, since being born is a blank state where learning begins. Most humans learn to associate the goodness feeling, to making others feel the same good, by the psychological empathy(ie being able to imagine themselves in a similar situation or even based off of real life situations). Doing the wrong things morally are also learned, and associated with the negative feeling, the one that is known as 'feeling bad'(which is different even if slightly in all humans).
Little kids, and babies, have no concept of mortal sin, that stealing will have dire consequences of eternal damnation or that lying is bad or wrong until their parental figure tells them so. At the same time there is a pragmatism to the way children work. The means that gets them their ends is the one that is most likely employed. Even studies of feral children in the wild is enough to prove that it is learned behavior of morality and it is not inherent.
Taboo is another interesting part of morality. When something is bad, there lies a feeling that taboo gives you that can also make you feel good. So when stealing may be wrong to most people, because they think about how they'd feel if it was them who was being robbed, when the association with the rush and good feeling of stealing becomes a motivation for that good feeling, they can by-pass their more rational thinking for the reward of that pleasure. It is also the same way fetishes are born. It is the whole, good feeling, for a specific stimulus and the associations built into the brain.
It seems that the way people are programmed(society and experiences with their internal representations) to act are two basic ways. They are either toward to- or they are away from-. Some peoples processes are motivated as towards to- success, toward to- feeling good. While others are away from- failing, away from- feeling bad. These emotional and logical motivators are never completely set in stone. A great example is religious dogma. Some christians are away from- hell while others are more towards-heaven. These differences in humans can be the difference say from one person asking you if you have heard of Jesus because they are toward to- heaven/virtue and you being there and others who are away from-hell/sin and are concern about you burning in hell and calling you a sinner.
This may help you to understand at least why I think this way. Based on the evidences of how people act and what is motivating them. This is where my problem with religion comes in. Because religion can take someone who already functions fine with the real world, and then imposes all these things to feel bad about, or good about, without any real evidence of why, other then people claiming a book says so or that the prophet of god says so.
To explain the last point further, the metaphor "The map is not the territory" is like this. Your own personal map, being that which you have learned and the knowledge you posses from your experiences(external and internal) is not the same as the real world is represented. When a foreign map of morality is then set in front of that person, there then becomes a conflict. Your map says X where the other map says Z. Very often when small children are impressionable they can be taught and often incorporate this other map as their own, changing how they experience the world around them. It is well known, that humans distort, delete and generalize in communication, and more often then not, it reflects what they do internally to represent that to themselves. Religious types tend to distort, delete, and generalize to very extreme levels. That is why it becomes so frustrating when talking to one about things of proven science to their impoverished maps of the world. Young earth creationists have distorted their map so much that look for evidence that what they believe is reality. Then delete anything that proves that what they believe is false, and they tend to generalize the type of people who do not agree with them as something negative compared to others who agree with them.
uhh im so tired right now, more later