Americans can go to Cuba under certain circumstances. Formerly, you would have to have a general license. Those were and are available to journalists, religious missions, cultural, sports - a few other categories. More recently, having significant Cuban family there would entitle you, as would being from Cuba.
Last spring, the current administration approved 'people to people' tours, run by properly licensed tour operators. These have just gone through a bit of a shakeup, with license renewals being held up as Rubio, in Florida, caused a stink about how the tours were being conducted. That's another thread and not relevant here.
The confusion is that the rules state (outside of the above exemptions) that Americans can go to Cuba - but they cannot spend money there. Since to enter the country involves paying various duties and buying health insurance, it automatically becomes illegal. One way around this is for the Cuban government to waive these fees, as they were going to do for the Sarasota-Havana race, or for a third party - Cuban or non-American - to pay all your costs for you.
So if you were sponsored by a Canadian like myself, you could go there. It would involve convincing OFAC (Office of Foreign Asset Controls) that that was indeed the case. And yes, paying your sponsor back IS a violation.
If you want to keep up with this, you can see my Cuba website, the link is in my signature, or join the Facebook page, http://facebook.com/cubaforbiddenparadise