What does it mean for a human-constructed system to be 'natural'? In most tribes of the world, the income of one person is not considered the property of that p

What does it mean for a human-constructed system to be 'natural'? In most tribes of the world, the income of one person is not considered the property of that person, but is divided according to customs about the distribution of wealth in that society. Tribal communities are about as close to 'natural' behaviour as you can get, so what is unnatural about it? I also don't understand the term 'contrary to reason'. How can a set of conventions be rational or irrational? They can be beneficial, harmful, fair, unfair, moral, immoral etc, but how can they be contrary to reason? Also: taxation is as much a part of capitalism as it is a part of socialism, especially taxation for expenditure on police and law.