That is not the point. Walking for miles would have been just as dangerous than staying put and wasn't an option for most. They had all the food, water, supplies within New Orleans (on dry land), more so than the small town they were kept out of.
A trick is being played here. There were tons upon tons of supplies in the city proper that was untouched and not utilized. The police did not keep the peace and there was a complete breakdown of local authority. How in heck would walking fifty miles going to improve their lot? The Mayor and Governor should be thrown in prison.
If you were black, and lived in NO, without a car, would you want to go to the superdome, or cross the bridge and be on high ground, and walk right into Bull Connor's office, and say high, got a place to stay? How bout letting me stay in one of them school buses? Or find a church or something. Or better yet, go to a hospital or medical facility. In any event, what they did is illegal. They should be sued, and then sued some more. Their lives should be made a living hell.