Moral Justification
The justification is really quite simple - all other taxation is on wealth created by individuals. In effect, all other taxation is theft of individual property. Taxation tends to reduce the amount of the subject of the tax, or at least reduces the incentive to produce the goods or services that are taxed.
But land value, or more correctly - the rental value of land, is created and increased by the growth of the community. It is only just that the landowner repay the community for the value it has created. The landowner does not create the value of land; he only appropriates it. Furthermore, unlike all other forms of taxation, regardless of the amount of the tax, the amount of land will not and cannot be reduced. In fact, the greater the tax rate, the greater the incentive to improve the land and put it to its highest free-market-determned use.
Land value taxation allows the community to recover the value it has created, while reducing or, preferably, eliminating the taxes on the goods, services, and wealth created by individuals. Every decrease in the taxation of wealth created by individuals is an increase in liberty.