FWIW, during evacuation/hurricane crises in Texas, this did NOT happen, at least not on any organized level. For one thing, Texas passed a law forbidding such confiscations. For another, Texas law regarding Official Oppression is quite broad in scope. Thirdly, Louisiana law is based on the Napoleonic Code, not English Common Law; one of the key differences is that the Napoleonic Code holds that one is guilty until proven innocent.
Now, being allowed into a shelter with weapons is a different matter. Those of use who want to evacuate with weapons had best be prepared. The owner of, or person in charge of the premises, can dictate what is allowed onto the premises. But, then, as someone recently posted on another forum, we prepare ahead of time, so we DON'T have to go to the Superdome.