You're right! I remember Shep saying they were stranded there.
Promising people that their property will still be there when they get back is probably the best incentive for them to evacuate.
It is likely that the people who are insisting on staying in NO despite the hideous conditions know that preserving property is the last priority of the authorities.
Although it seems harsh, the Gretna police strategy would have dovetailed with the purported N.O. evacuation plans, had that plan been implemented. The evacuees would have ridden out on buses, not walked.
It is hard to blame the Gretna police for not sacrificing their city just because New Orleans did not execute their plan.