Duty Free Cigars

Ok, so I have a few questions regarding Duty Free.

I am sitting in Miami International, heading to Grand Cayman, and I went to the Duty Free Americas here and saw that the davidoff 4 packs were as much or more than the local B&Ms, why is that? Shouldn't Duty Free be cheaper.

Secondly (if this question is inappropriate, please just edit it down), how does Duty Free work coming back into the states. If I buy some cigars from another island in the Duty Free, will it flag me when I get to customs?

Thanks gang!!!