i am unaware that it is illegal for an american to go to cuba.
in fact americans have gone to cuba many times. just there are no direct flights from the us to cuba, and the us govt does not have a visa agreement with cuba, nor a consulate.
Well State puts it this way....
The Cuban Assets Control Regulations are enforced by the U.S. Department of the Treasury and affect all U.S. citizens and permanent residents wherever they are located, all people.....ya ya...everyone.
The regulations require that persons subject to U.S. jurisdiction be licensed in order to engage in any travel-related transactions pursuant to travel to, from, and within Cuba, or that the transactions in question be exempt from licensing requirements.
Transactions related to tourist travel are not licensable.
This restriction includes tourist travel to Cuba from or through a third country such as Mexico or Canada. U.S. law enforcement authorities enforce these regulations at U.S. airports and pre-clearance facilities in third countries. Travelers who fail to comply with Department of the Treasury regulations could face civil penalties and criminal prosecution upon return to the United States
http://travel.state.gov/travel/cis_pa_tw/cis/cis_1097.html#entry_requirements
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So, in particular I know from my sailing days in the Gulf, the Coast Guard is very practical. Water, provisions, fuel?
"Wha'cha doin' out here?" (don't glance at the 50s, don't glance at the 50s) "Looks like the Gulf Stream took us and we are lost!" And that was more or less true. But had plenty of supplies.
No US citizen, unlicensed for such activities, may conduct transactions with Cubans in Cuba or Cuban waters.
"How much do those fuel tanks hold, Sir?"
"When did you last re-fuel?
May we see your charts?"
"Do you Coasties have a carburettor or a stiff breeze home? We can't keep the engine running."
So, they were very nice after they shook down the boat for drugs..ahmm...conducted a safety equipment inspection. And sent a bosun over to look at the engine...(see if he could smell any dope) He poke thru the bottom, where we had the carb off of the intake manifold. That was the water jacket rusted thru and replacing the fuel in the float bowl.
"Yep, there's the problem."
Nothing they can do but cast off. We waited for the wind. And I spent the first day in a week not working on that motor.
Beyonce went on some organized visa waiver or something. Being a guest of the country is not considered completing transactions, I imagine.