Update: See Paul's comment below about why they had to do what they did, and what they're doing about it.
Are there quiet, law-abiding people on Sixth Street? Sure, but it's hard to stand out when your neighborhood looks like something out of Mad Max: Beyond Thunderdome -- take a walk down 6th, especially at night, and you'll find yourself surrounded by brawls, the mentally ill, drug deals, screaming gibberish, glazed-over eyes, and people lurching unpredictably or just leaning against whatever they can find, waiting around for who knows what.
Sixth is a magnet for the unlucky poor. Some of them might benefit from rehab and job training, but many would not -- their damage is permanent, the result of brain-corroding drug addition or mental illness, and often both. In a perfect world, someone would pay for them to be institutionalized; but for these folks, the money isn't there. And even if it was, you can't lock up someone who wants to be free; and besides, the state's too busy buying unworn suits to pay for lifelong inpatient care. So until they pass away, they've got to go somewhere.
Organizations like The Tenderloin Housing Clinic cluster on 6th, serving as containment for many of these folks. On 6th, the THC is distant from most citizens' daily destinations -- so people who can't take care of themselves have a place to loiter, and most San Franciscans never even see them.
Until a year ago, when a guy named Jeff set out to raise up 6th Street's visibility. His jarring footage was drawing lots of eyes -- but now the THC has set out to stop him.