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.
That’s what I think too. A law where you pay more tax for additional properties you own on top of your first home. I don’t know enough about the housing market to know if such a law would actually help, but at first glance it does seem like it’d break up some of the corporate slum lord businesses around the US