Originally Posted by
EUnomad
How could they? US law is not applicable in Cuba. You would probably have to get permission to book a ticket from the US to Cuba, so no one could fly from the US to Cuba, and those who do two legs are outside US jurisdiction.
A lawyer might give a better answer, but if the OP flies from Canada to Cuba, theoretically he could announce it on youtube with video saying in-your-face-Bush, and get a cuba stamp on his US passport. He would be in the clear because he was wholly outside US jurisdiction, where US statutes do not apply (as the US was not the departure point).
I'm a lawyer so I will give a better answer (note: not legal advice). US law can apply to US citizens anywhere (and can even apply to non-citizens outside the US in certain circumstances, such as anti-terrorism laws and securities laws).
The key point is whether the US can or will enforce the law. The US government has extremely limited law enforcement monitoring resources outside the US, and also usually cannot go to a foreign country to arrest you unless they team up with the local government (which implies that you have to be breaking local law as well).
However, if you were to go on YouTube and publicize your presence in Cuba, and if a federal agent came across the video and had a mandate to enforce the law, you could easily be stopped, arrested and slapped with fines the moment you step off of your returning flight in the US. Even if you never set foot in the US again, the government could theoretically seize any assets you left behind in the US (including cash in American bank accounts) while you are gone. This probably doesn't happen often because most people daring enough to break the embargo are discreet about it.