On a domestic flight the passenger could get off the flight as long as they could do so safely. I was enroute from Baltimore to Houston not too long ago and diverted to New Orleans for weather. Two passengers happened to have New Orleans as their final destination but were scheduled to connect through Houston. They were thrilled when we allowed them off the plane and they got home 4 hours early. Their luggage however stayed on the plane as we had no way of retrieving it. It continued onto Houston and arrived back in New Orleans later that day. Any security concerns about customers leaving their bags on the plane in this situation can be rationalized in a common sense way. That is, the passengers had no idea we were diverting to New Orleans when they boarded so no big deal. On International flights the passenger(s) would need to deal with customs and immigration which would likely cause a problem with them leaving the airplane.