That US Customs link has been around and around. The key is:
It is also illegal for U.S. persons to buy, sell, trade, or otherwise engage in transactions involving illegally-imported Cuban cigars.
What is impossible for anyone to prove is that any Cuban cigars in anyone's possession were obtained by being imported illegally. Also, who's to say they're REAL anyway? Anyone can buy fake bands and put them on cigars made anywhere in the world, it happens all the time (just ask Oblik )
One more thing about that link, it's not quoted verbatium from the "Trading with the Enemy Act" it's a bureaucratic "scare" intepretation of the Act by Customs which is only their view.