Getting serious on this one....
If we are getting into a legal discussion, better be a bit clearer (and more serious): the Law of the Sea is a body of case law and treaties, originating from the 17th century book by the legal scholar Hugo de Groot and generally adhered to by 'civilized' nations throuhgout the ensuing centuries (when it suited them, probably). It is now codified in the United Nations Convention on the Law of the Seas, which has International Treaty status and as far as I know is ratified by almost all UN member nations, and includes specific tribunals to deal with issues etc. etc.It also deals with Piracy and actually defines piracy. Being serious now, I must make the point that boarding and detaining suspected pirates/vessels under control of pirates is restricted to government vessels.Where does the Jolly Roger come in? It can still, based on case law, be seen as an intention to commit acts of piracy. However, the Law of the Seas applies on the 'High Seas' and within the territorial waters national legislation takes precedence. In the absence of any national legislation, though, one could argue that therefore the Law of the Seas applies or can be applied.Hence the 'unamused' Coast Guard, who obviously are well aware of these (international) laws, and probably think, rightly or wrongly, that flying the Jolly Roger is silly. I don't know about US law, but outside the 3NM zone, on the strength of the Law of the Seas, the US COast Guard would be entitled to board a yacht that flies the Jolly Roger to ascertain themselves of the real intentions. What happens next is up to litigation lawyers.....