I thought that it technically is not illegal to enter Cuba physically; it is just illegal for persons subject to US jurisdiction to spend any money in or related to Cuba without explicit authorization from the Treasury Dept. (So, for example, there are some exceptions for "fully-hosted" guests of Cuban families, mostly their own family members visiting and so forth, who must agree not to spend a penny to enrich a Communist country. Not sure if that is an archaic interpretation, but that is what I interpret the purpose of the sanctions to be.)


Here is the official wording:

...regulations require that persons subject to U.S. jurisdiction be licensed in order to engage in any travel-related transactions pursuant to travel to, from, and within Cuba, or that the transactions in question be exempt from licensing requirements. Transactions related to tourist travel are not licensable. This restriction includes tourist travel to Cuba from or through a third country such as Mexico or Canada. U.S. law enforcement authorities enforce these regulations at U.S. airports and pre-clearance facilities in third countries. Travelers who fail to comply with Department of the Treasury regulations could face civil penalties and criminal prosecution upon return to the United States.

Source:
http://travel.state.gov/travel/cis_p.../cis_1097.html