The city was fully aware of the fact that they had a mass of drug addicted criminals who would seek shelter there- because those are the exact people who don't have cars. The city poured that element in with innocent poor families and tourists and sick elderly- if they were paying attention and had thought about it ahead of time- they would have predicted this disaster to the T.I'm not sure what the alternatives were - I didn't hear anybody criticize the choice of the Superdome except for people worried the roof would not hold.
Also I thought FEMA actually set up the shelter and I remember those were national guard troops searching everyone before they were allowed in.