The reason is because it depends on your definition of "Federal Law"
Typically, Federal Law means "codified" (in the US Code) The Constitution is not 'codified law', but is the basis for all codified law.
My full answer is here that provides 'yes' and 'no' responses that depend on your definition of the question you were asking:
http://tigerboard.com/boards/view.php?message=14350729Edited by Spanky at 12:45:48 on 12/06/16