Many of the recently-completed and currently under construction building projects are mixed-use buildings with retail on the ground floor and condos above. The ground floor often has floor-to-ceiling glass. It turns out that many of these ground-floor commercial units are actually empty.
Fines & other regulation
As of Aug, 2014, "owners of any storefront left vacant for more than 270 days to pay $765 annually and register with The City." [source]
Previously, a 2009 law made it so that "owners of vacant buildings are required to register their properties and pay a fee, as well as secure the grounds and provide insurance. But if the building is only vacant in the storefront and occupied above, those rules don't apply." [source] But because so many of the new buildings have occupied condos above, the rule didn't apply. The Aug, 2014 rule closes this loophole.
Apparently the city hates vacant storefronts: "In 2009, empty storefronts were such a plague that The City got a little creative by launching an Art in Storefronts pilot program to try and bring a little life to the shuttered spaces in the Mid-Market and Tenderloin neighborhoods." [source]