> what is the government's justification for taxing the value of land I own outright?

In the 1970's a group of people decided to setup a micro nation on a small unclaimed island. A nearby country Tonga seeing they where not protected by another state invaded, so ended the micro nation.

Ultimately it's the US governments providing a military and police force that causes a piece of paper to have any impact on who controls a piece of land. Such services require payment. Though we charge for local services ie schools, EMS based with property taxes and protection FBI/Army based on people (income tax). Historically this favors land owners, but we could swap in the future.