There's more than one kind of troop there. Nagin/Blanco have complete control of the National Guard. Have had ALL ALONG.
The active duty military (Honore's command) is provided only for evacuation, see air bridge on Saturday last. They may also do some search and rescue, and will place relief supplies, etc. But they will NOT perform any law enforcement whatsoever. It's illegal.
Nagin can have as many Nat'l Guard as he needs to cause forcible evacuation - all he has to do is call for them and give them orders.
Nobody told Nagin apparently since he thought he could order Gen. Honore's troops as well.