Based on fiduciary responsibility to current Square shareholders, I can't believe he's allowed to do this.
Also, the CFO is probably just as important if not more for getting the company ready for IPO.
Also the above commenter's point, talking to people 16 hours a day does not mean you're working. An engineer's work is quantifiable, so they can be held accountable when they claim they work 16 hours a day. Not so for a manager. Their effect (good or bad) is far more difficult to quantify.
edit: Alternatively, he could live in the mission and then walk between the 2 offices during the day, they're right next to each other.
The Tenderloin's on the opposite side of city hall.
Then again, somehow he's doing it.