The city of SF is not enforcing anything - it's the landlords. In SF, most units are covered by rent control, meaning most people are paying rents far below the market value. Landlord are prohibited from increasing rents or kicking out current tenets unless they violate their lease. So any lease violation, such as subleasing, can be used as an excuse to evict the tenet and get one that will pay the current market value.
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The city of SF is not enforcing anything - it's the landlords. In SF, most units are covered by rent control, meaning most people are paying rents far below the market value. Landlord are prohibited from increasing rents or kicking out current tenets unless they violate their lease. So any lease violation, such as subleasing, can be used as an excuse to evict the tenet and get one that will pay the current market value.