A rented apartment is a liability to the tenant, but an asset to the landlord. And whether a residence is rented or owner-occupied isn't an inherent property of the residence, since it can change over time–it isn't uncommon (for a person of sufficient means) to buy a new house/apartment/etc and then rent out the old one instead of selling it.
> We generally call them "condos" when owned.
Is this a peculiarity of American English? In Australian English, "apartment" is about the type of physical structure, and has nothing to do with the building's legal structure. (What Americans call "condominiums", Australians call "strata title", but it is essentially the same thing–newer apartment buildings are almost all strata title, but older buildings sometimes have other ownership structures, for example company title.)