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’s pretty common to have a building that’s partially inhabited by renters and partially by owners. It’s not super unusual to have a building owner sell only some units in a building.
It’s entirely possible to put a clause in the contracts associated with the sale that the new owner must give the old owner a share of any profits on a resale within five years.
If there’s a building with five rent controlled units, two of which are occupied, and the owner of the building sells off the other three, the two occupied units will continue to be rent controlled and belong to the landlord. If the landlord doesn’t want them and lowers the price by a bunch or makes a special offer to the tenants, people still don’t have to buy it. If people start to think being a landlord is risky and maybe not a great idea, good.