Just a guess...maybe because rescue choppers need the airspace to get people off of roofs and out of trees?
That's what I'm thinking. The airspace over New Orleans is probably crowded right about now (and probably getting more so), and there's almost certainly no low-level air traffic control to speak of, if both airports are still knocked out (meaning no control towers). It's strictly see-and-avoid, and the fewer sightseers up there, the better.
That having been said, maybe those news helos should be pressed into use to move evacuees and supplies. Not doing the hardcore rescue work that the police and military are trained for, but shuttling folks or supplies around as needed.
}:-)4