Rent controlled apartments being held by tenants no longer using them as primary residences is pretty common. A famous case of this was Cleve Jones in San Francisco who tried to make it a huge political deal when his landlord raised his rent to market because he was 1. living in Guerneville full time and 2. subletting the apartment. The media environment is one where it's ok for a master tenant to be a de facto landlord and make money on real estate, as long as it's not the landowner themselves!