While I was a BART commuter, I walked a gauntlet of slumped-over hard drug users, sprawled-out homeless people, human urine, and feces every day. Fortunately, I've only seen people drop their pants and take a shit on the sidewalk in front of me twice. Missed a random stabbing on the stairwell into my station by about 5 minutes.
This is Civic Center Station, which serves Twitter, Uber, and Square. No amount of tech worker foot traffic is going to clean up an area if the government isn't willing to police it.
Video doesn't even show the worst part, but helps to illustrate: https://www.youtube.com/watch?v=5gT5NULvRSk