There is probably some truth to this. Everything in Louisiana is controlled by unions.
"In an evacuation, buses would be dispatched along their regular routes throughout the city to pick up people and go to the Superdome, which would be used as a staging area. From there, people would be taken out of the city to shelters to the north.
Some experts familiar with the plans say they won’t work.
... It’s also unclear whether the city’s entire staff of bus drivers will remain. A union spokesman said that while drivers are aware of the plan, the union contract lacks a provision requiring them to stay."
http://www.nola.com/printer/printer.ssf?/washingaway/leftbehind_4.html