
Actually, its not
Case law has shown that free Americans can go anywhere they like (Lawyer uncle looked this up for me). What kind of government restricts where its citizens can go?
It is, however, illegal to engage in commerce of any sort - IOW, no trade with Cuba. Its the embargo (or as the Cubans call it - blockade).
Now the tricky bit is proving you didn't trade dollars for anything while in Cuba. No meals, no drinks, no fees of any kind. A few cruisers on sail boats have visited Cuba and "self hosted" (they say) - which is to say - only ate food they brought, drank rum they brought, and only walked around - no trade.
Its getting harder to prove that though and the pitbull feds are turning up the heat on these people when they catch them to try to make them examples and discourage the rest of us from going there.
There sure were a lot of boats flying US flags at Marina Hemingway New Years Eve tho...
The whole Cuban situation is a microcosm of what's wrong with US foreign policy. Its the best example we have of how embargos and economic sanctions are ineffective, how not all dicatators are evil (Castro seems to be doing alright by his people given his constraints).
The idea of using illegally occupied territory rightfully belonging to a country whose government we've been trying to topple for almost 40 years to hold prisoners from another war is just too much to take.
I think that it's extraordinarily important that we in computer science keep fun in computing. When it started out, it was an awful lot of fun. Of course, the paying customer got shafted every now and then, and after a while we began to take their complaints seriously. We began to feel as if we really were responsible for the successful, error-free perfect use of these machines. I don't think we are. I think we're responsible for stretching them, setting them off in new directions, and keeping fun in the house. I hope the field of computer science never loses its sense of fun. Above all, I hope we don't become missionaries. Don't feel as if you're Bible salesmen. The world has too many of those already. What you know about computing other people will learn. Don't feel as if the key to successful computing is only in your hands. What's in your hands, I think and hope, is intelligence: the ability to see the machine as more than when you were first led up to it, that you can make it more.
--Alan Perlis