Not true really. There are a number of federal laws that make it possible to eliminate the Governor of Louisiana from all decisions -- effectively removing her from office. A presidential declaration of Martial Law in Lousiana is only one mechanism by which the Governor of a state can be effectively neutralized. And the new powers of Homeland Security and FEMA really can swee a governor into a closet while they do what they think needs to be done.
Martial law doesn't typically push the gov out. As for the new Homeland and FEMA powers, I'm totally in the dark and know absolutely nothing about them. Thanks for pointing that out. It totally escaped me.