The argument would be that communism/socialism is not the root cause of the embargo rather it's a peripheral cause. The root cause is not incompetence on the part of the Cuban government but malice and while communism/socialism may be a red flag :-) for malice it's not why the embargo exists.

Now is that true? I don't know but I'd bet that's more or less what you'd hear from a supporter and the easiest move is to just take them at their word.