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.
Who says all apartments are supposed to be used for low income housing?
The problem with low income housing is the NIMBY’s, and the very real fact that when you have large amounts of low income housing you get a low socioeconomic area along with all the problems that brings.