For good or bad the people of New Orleans elected Nagin as mayor and the people of Louisiana elected Blanco as governor. Bush simply cannot go around the country pushing aside incompetent Democrats and taking over without them asking him. I can't even imagine the Democrats' furor, they would be saying we had a dictator if he had rushed in either before or right after the hurricane.
You're assuming they'd be upset over Bush taking this out of their hands. I don't know if that's the case. Maybe you're right and they would. But, if they wouldn't mind, a lot of problems could have been solved before they even developed. I'm curious if Bush asked them.