Because the latter is not a thing. The United States does not implement any border firewalls on traffic entering the country. No law compels blocking Cuban citizens from accessing US hosted content, just preventing them from entering into financial transactions.
I honestly have no idea how the sanctions are reflected in the actual wording of the law, but what I see is that many companies are actually overzealous and wholesale limit (block) access regardless of the outcome of the content request (that is, even if the transaction is not a financial one.)
Let me give you an example: I can't open dell.com, at all. What I want to believe is that they blocked all access because it was easy, just a geo thingy flipped on. It is their decision though, but it is supported on existing sanctions. So... yes, the law compels them to do it, indirectly or not. And there are hundreds, thousands of other examples that I can provide, if you're interested.
