The laws regarding the importation of any Cuban products - including cigars - into the US are unambiguous and harsh. However, I do not think that they are enforced with any vigor.
These laws seem to be a testament to the political power of the Cuban emigree population in Florida, who (rightly) hate Fidel so much that they are (wrongly, I feel) willing to harm all Cubans economically.
Re cigars, the history is that, prior to the embargo in the 60s, it was Cuban tobacco that was in demand - not Cuban cigars (which were a small part of the market). The best hand-made cigars were built with Cuban tobacco fillers and Connecticut wrappers in Miami, Tampa, and New Jersey.
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Editor: Juan Paxety corrects some of The B's assumptions, in the comments.