They are covered by a special tourist licence from the Jamaica Tourist Board. They are there to serve ONLY tourists with goods that are not made in Jamaica.
They are not really free of duty as they pay various fees and taxes, plus the freight, to bring the items here.
These goods are usually European or Asian in origin and sold to Americans who would otherwise have to travel long distances for them.
Over the past few years, the internet has been giving them a hard run for their money as you can get the same, similarly priced, goods delivered straight to your door. Also, with airline prices increasing, tourists are finding it more expensive to travel.