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.
Income tax when you aren’t receiving an income is a weird idea.
It sounds like the author wants a land tax, but hasn’t ever heard the term.
Another term is a vacant homes tax, something Vancouver and Toronto in Canada are using.