Who ever said government policies had to make sense, Part II
While I'm not certain, I think it may be prohibited for Americans to travel to Iran and North Korea, too. I don't have time to research that point right now.
The restrictions on travel to Cuba has been in place for at least 40 years, at least as far back as the JFK administration. The goal is to limit trade with Cuba, therefore weakening their economy. Since other nations have not followed suit, the restrictions are essentially meaningless to undermining Castro. By the same token, if they're meaningless, then what's the problem? Surely no one can rationally claim that a single country refusing to trade with Cuba represents an undue hardship on them.