I've always suspected the American embargo was more motivated by the loss of American property during the Cuban Revolution and that initially it was more about exploiting the opinions of the Cuban refugees to further their ideals than the other way round. I mean, the significant Cuban population in the US wouldn't exist if it wasn't for laws that allowed them to migrate somewhat more easily after the Revolution. In a sense, the large Cuban-American population is a result of American policy on Cuba rather than the other way around.
Many Americans and American companies lost a substantial amount of money and property after the Revolution. The government of the United States must have - at some point - realized that it would be hugely hypocrticial of the American government to punish Cuba and nations/companies dealing with Cuba when the US's own founding revolution robbed many of their property (the United Empire Loyalists being chief among these). In fact, the Helms-Burton Act was responded to in Canada with the
Godfrey-Miliken Bill - a rather interesting moment in Canadian-American relations regardless of how you feel about it.
The relationship between Cuba and the United States is extremely complicated and goes back far before the revolution ever occured. There's more to this embargo than just Communism vs. Capitalism or a fear of attack. If it was based solely on fear of attack, wouldn't the US have dropped the embargo when the USSR fell apart and that fear disintegrated?