as long as you are departing and/or arriving from the USA, you're subject to their jurisdiction, regardless of your nationality... like in any other country as well, as long as you dont have a certain status of imunnity..
this means if you want to fly directly from the US to Cuba, the only legal reasons to do so, are:
family visits; official business of the U.S. government, foreign governments, and certain intergovernmental organizations; journalistic activity; professional research and professional meetings; educational activities; religious activities; public performances, clinics, workshops, athletic and other competitions, and exhibitions; support for the Cuban people; humanitarian projects; activities of private foundations or research or educational institutes; exportation, importation, or transmission of information or informational materials; and certain authorized export transactions.
(Beach-) holidays do not fall under one of these categories.... each of these categories is well defined describing what is expected to be done... (just cant find it online .-( )
and family visit is easy to check: either you travel on a cuban passport, or having cuban family visa or you can show that you have close relatives living in Cuba...
The Cubans dont care, from where you're coming... it's an US thing..