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.
Because it’s a place to live in without having rent being arbitrarily dictated by some random dude. It’s instead a tax you pay based on who you vote for.