WHAT'S DUTY FREE
Duty Free means that no taxes have been added to the cost of the merchandise.  Merchandise purchased from duty free stores, is free of duty and tax in the country in which the duty free store is located. All tax-free merchandise purchased from a duty free store must be exported from the country of purchase.  These items are not meant to be used, worn, eaten, drunk, etc., in the country of purchase.

Articles purchased in American duty free shops will be subject to U.S. duties if you bring them into the United States.  Shopping tax-free means significant savings because the duty free retailer is able to offer merchandise at lower prices.