As an old resident of San Francisco, who goes back at least once a year (and I will be there again this month), I assure you this sort of incident is not unusual.
It was, once, VERY unusual. Armed robbery on Union Square just wasnt done. Now?
Yes. Like most large American cities now, it was once a problem that was managed by policing, but now...criminals in most of those places have free reign.
It must break your heart to see what that city has become.