They didn't just walk out of the city and into dry ground where they could have gotten water and food because the authorities didn't let them. And this was after one group of enterprising people pooled their resources and ordered a fleet of busses to come to new Orleans and get them (from a paramedics convention) but had those busses commandeering by the military when they hit city limits. So they walked en mass to a police station that lied and told them they could walk across that bridge, to get them to stop being encamped by the station in the hundreds.
In the middle there is a story about how part of the group organized a small group of maybe 90 people who got organized and were living on an unused section of freeway (on the approach to that bridge, as I understand it) in cobbled together shelter that the military destroyed to get them to disperse as though they were a mob.
This is... this is all so sad.