Why? Because apparently they need some more incentive to keep units occupied. Also, even though a property might be vacant, there’s still imputed rental income there. Its owner is just receiving it in the form of enjoying the unit for himself instead of receiving an actual rent check from a tenant. That imputed rent ought to be taxed like any other income.
I’m not sure what you’re getting at. My property tax bill has separate components for the land and the improvements upon it. I’m sure that a larger portion of my income goes to paying it than some of my neighbors.