Maybe these people have their own houses and vehicles and families to worry about without having to bailout Nagin and the rest of the city over its non-plan. If there were an actual plan, those drivers who would participate in evacuating would likely also have a means of getting their needs taken care of, too, knowing they would be busy evacuating other people at the same time.
I don't blame the school bus drivers for getting out of Dodge. No one told them to stick around. The mayor, in the worst case, should have/could have rounded up other city employees to drive at least some of the buses in this case. But with no plan and no support from the mayor, I'd leave town, too.
Because they had no official authority to do so, due wholly and completely to the negligence and dithering of the Orleans Parish School Board.
Nagin also had no authority to use the buses except under declaration of emergency, a problem the City had been trying to resolve in its negotiations with the (stonewalling) School Board.