"I was under the impression they won't be allowed to stay."
As I understand it, the plan is to allow people in, get their things, and then get out for at least three months.
Also, the Plan is to allow nobody in without ID and proof of address, I hear.
That's right, I forgot about the ID part. I'm sure that's a good thing.
That is the standard Hurricane Evacuation rule. If you leave, you must have proof of residence to return in the event of damage/destruction. It keeps out the gawking lookie loos.