In case anyone wonders how long ISS could survive completely unattended - it averaged 1 debris avoidance manouver per year before 2011 and 4 in 2012. Its orbit is around 400km which is low enough do decay slowly, but boost burns happen only every few years. Both of those events are planned months or years ahead.

NASA must have decided there was no real danger to the operators apart from some longer shifts and sleeping in the office. Otherwise, even without the backup ground control, ISS wouldn't be in any significant danger if disconnected for a few days.