The thing of it to me more than anything comes down to an "I", "Us", and "them" perspective. This is inherent in all societies and is typical of humans and many animals. You could even think of it in terms of individual, pack, and prey. I don't necesarrily consider these moors and taboo to necessarily be parasitical in nature, more times than not they are more symbiotic and necessary for the survival of this or that species/subgroup.
First off, the individual has only himself to think of. He can devise any system of survival that suits his style and path. This can all be done without thought for anything or anyone but him<her>self. It is very easy for an individual to reach a state of equilibrium with his<her> enviorment to live out a fairly decent life. You can see examples of this of various individuals that have been stranded in the great wildernesses <perspectively> around the world. But this is only true when not having to deal with other peoples. The more people in the equation, the harder it is to acheive that equilbrium. One reason for this is that it is easy with a group of people for an individual to become "them".
With the "us" or pack <herd, gaggle, covey, pride...or any other subgrouping of critters> it is a bit harder to acheive a state of balance because you have various ideas to achieve survival. Generally speaking the group will form a set of core ideals and follow those ideals. These ideals are necesarry for survival for the group. Without a form of harmony within the group, you swiftly have a break up of the group and either becomeing a bunch of individuals doing what they can to survive, or two groups forming and going off on thier own way. Within a limited resource scenario this can be quite badand natural selection tending to say who gets what. what also forms because of this schism is the "them" or prey denotation. If you are not part of the group, then you are against them or seen as a resource to be preyed upon. Once again natural selection being what decides which group will be the victor. You can see this throughout nature and it is a very valid arguement for having various social biases. This being not an arguement about moral right, but rather an arguement about the necessity of having unity within a group to insure survivability. Through unity you have strength.
Now this may or may not have biological standing, I don't know for sure. In essence whenever you have a society collapse it is because of the differences between one group and another fighting to survive. Once again this is not about moral right<though many tend to cloud the issue with that>, but rather about survival of a group. When survival of you and yours comes into play moral right can be tossed out the window ina hurry else you die. Think about it a bit, you may not agree it is right but you know that it is what happens.