I don't really think Castro has been a threat to the US since the Cuban Missile Crisis in the early 60's. But foriegn policy denotes, keep your friends close, and your enemies closer... And as far as China is concerned, we don't need to fuck with them anymore than we need to lift sanctions from Cuba. Although trade embargoes from the US have hurt the Cuban economy they still have enough trade partners elsewhere in the world to where it really doesn't make a difference. The poverty in that country has only been brought on by Castro's policies, and the hugely unbalanced distribution of wealth caused by those policies...
'...Smoking's bad, smoking killed my dad. Yeah, he was driving down the highway one day and as he was lighting his cigarette, it blew out the window. So he jumps out the door to save it, and ran himself over. Uh, you mind if I smoke?' -Olie Ollaussen, Ski Bum extraordinaire