the regulations describing how Americans can travel to Cuba - it's tightly restricted. It's technically not illegal for an American to travel
to Cuba, but it's illegal for an American to spend money
in Cuba without a "license." That's why the regulations are issued by Treasury rather than State - the Office of Foreign Asset Control comes under Treasury. As a practical matter, an American can't even travel to Cuba even if they took food and water with them and camped out on the beach - the moment the (Canadian, Mexican, whatever) airline paid the Cuban government taxes related to the American's ticket (even if bought outside the US), the law's been violated.
I don't think it would even be legal for a US airline to offer non-American members of their FF
program opportunities to redeem miles to Cuba - that'd be facilitating transactions "in which Cuba has an interest", since presumably the non-American traveler would spend money in Cuba.
(Yes, it's ridiculous that an American can travel to, say, China, whose government is just as repressive as Cuba's, but not to Cuba. Don't get me wrong, it'll be a great day for the world when Fidel Castro assumes room temperature, but I just don't see how Cuba's government is any worse than that of many other countries Americans can travel to at will.)