As far as I know, visiting Cuba without a permission from [U.S. Department of the Treasury][1] is illegal. 

**Title 31--Money and Finance: Treasury<br>
CHAPTER V--OFFICE OF FOREIGN ASSETS CONTROL, DEPARTMENT OF THE TREASURY<br>
[PART 515--CUBAN ASSETS CONTROL REGULATIONS][2]**
![enter image description here][3]

Even if you do have a permission, some restrictions still apply, as stated in this document ([CUBA TRAVEL ADVISORY][4]):

> Authorized travelers to Cuba are subject to daily spending limits and
> are prohibited from bringing any Cuban “souvenirs” or other goods into
> the United States, with the exception of information and informational
> materials.
> 
> Civil and criminal penalties may result from a violation of the
> Regulations.


  [1]: http://www.treasury.gov/resource-center/sanctions/Programs/pages/cuba.aspx
  [2]: http://edocket.access.gpo.gov/cfr_2010/julqtr/pdf/31cfr515.420.pdf
  [3]: https://i.sstatic.net/wmf7b.png
  [4]: http://www.treasury.gov/resource-center/sanctions/Programs/Documents/cuba_trav_adv.pdf