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.
It is possible to distinguish between home occupiers and renters, and were a law made like this some method of distinction would be needed.
The entire purpose of a law like this would be to force renters to keep their lots occupied or sell them if they cannot afford to do so. Either way, you either see a drop in rent costs from a market increase, or you see a drop in real estate costs as the market increases. And either way you do not have tens of thousands of vacant homes while homelessness is on the rise.