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.
No outrage here. I’m using the word enjoy in an economic sense, as in possessing something and getting some benefit out of it. The concept is called “imputed rent” if you’d like to look it up.