Because they are historical landmarks. This is nothing new. There are cases of private homes that are declared as historical landmarks that are still funded my the state. Take for example some of the historical homes in the washington DC area. Some still have families living in them, yet when major renovations and restoration projects are neccessary, tax payers flip the bill. They do this to insure that no matter who owns the landmark, the landmark is well taken care of for future generations. If we are expected to flip the bill for other privatly owned historical structures why would these missions be any different?

-Chris