From the US State Department website:

The following categories of travelers are permitted to spend money for Cuban travel and to engage in other transactions directly incident to the purpose of their travel under a general license, without the need to obtain special permission from the U.S. Treasury Department:

- U.S. and foreign government officials traveling on official business, including representatives of international organizations of which the U.S. is a member;

- Journalists and supporting broadcasting or technical personnel regularly employed by a news reporting organization;

- Persons making a once-a-year visit to close family relatives in circumstances of humanitarian need;

- Full-time professionals whose travel transactions are directly related to professional research in their professional areas, provided that their research: (1) is of a noncommercial academic nature; (2) comprises a full work schedule in Cuba, and (3) has a substantial likelihood of public dissemination;

- Full-time professionals whose travel transactions are directly related to attendance at professional meetings or conferences in Cuba organized by an international professional organization, institution, or association that regularly sponsors such meetings or conferences in other countries;

- Amateur or semi-professional athletes or teams traveling to Cuba to participate in an athletic competition held under the auspices of the relevant international sports federation.

The Department of the Treasury may issue licenses on a case-by-case basis authorizing Cuba travel-related transactions directly incident to marketing, sales negotiation, accompanied delivery, and servicing of exports and reexports that appear consistent with the licensing policy of the Department of Commerce. The sectors in which U.S. citizens may sell and service products to Cuba include agricultural commodities, telecommunications activities, medicine, and medical devices. The Treasury Department will also consider requests for specific licenses for humanitarian travel not covered by the general license, educational exchanges, and religious activities by individuals or groups affiliated with a religious organization.

Unless otherwise exempted or authorized, any person subject to U.S. jurisdiction who engages in any travel-related transaction in Cuba violates the regulations. Persons not licensed to engage in travel-related transactions may travel to Cuba without violating the regulations only if all Cuba-related expenses are covered by a person not subject to U.S. jurisdiction and provided that the traveler does not provide any service to Cuba or a Cuban national. Such travel is called "fully-hosted" travel. Such travel may not by made on a Cuban carrier or aboard a direct flight between the United States and Cuba.

Failure to comply with Department of Treasury regulations may result in civil penalties and criminal prosecution upon return to the United States.

The State Department often issues travel warnings that discourages Americans from visiting many countries where they may be in danger. When someone insists on travelling to a place like Afghanistan or Burundi, they shouldn't cry for the Marines to come pull their asses out of the fire if (when) they get into trouble.