Certainly in times of such crazy rental rates (which are lower than they used to be but not by much) it's frustrating to see perfectly nice residential properties sit vacant while people remain homeless.
Enter Homes Not Jails, an 18-year-old housing advocacy group which likely organized this protest in January up Market Street note the same signage, "House keys not handcuffs." They've been taking over homes in this fashion for well over ten years, and they're affiliated with the SF Tenants Union.
The group spearheaded an action on Sunday in which they invaded a vacant property on the 500 block of San Jose Street that was the site of an Ellis Act eviction in 2008 that left an 80-year-old man, who had lived there 43 years, virtually homeless (he now lives somewhere in an office building in the Mission). The owner of the building, Ara Tehlirian, denies that the building sits vacant and says she uses it for her own "personal purposes." While that might be debatable, Tehlirian's lawyer makes the valid point that "It's sort of ridiculous to think that a private property owner like Mr. Tehlirian would have any obligation to house the homeless."
But yeah, it is kind of shitty if she evicted this old man and still can't fill the place.