"It is a hobby in Axis to study efficient taxation - that is, taxation that does not discourage that which you wish to encourage. Taxes on trade goods, for instance, are an obvious necessity in the Material because no one would maintain the roads without their fair cut but they do make trade more expensive, and so the lord tilts the scales against the very thing he most desires. By contrast, a tax on a trait that cannot or cannot easily be changed - such as being a tiefling, or possessed with unusually fair features - alters no one's conduct. This makes the latter kind of tax superior, where it can be employed. It discourages no one from their natural pursuits, while still increasing revenue. I don't find this logic applicable to tieflings as we ought to be rid of them entirely, but for orcs or halflings, say, I think it follows."