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.
If you’re that kind of person then you’re already charging as much as the market will bear. I think you would have an easier time doing what you can to fill the vacancies or just selling the vacant units.