It is indeed (almost) legal. Under the Consittution of the United States, our Congress has the power, "To define and punish piracies and felonies committed on the high seas, and offenses against the law of nations. The United States Congress did pass such a statute TITLE 18 > PART I > CHAPTER 81 > § 1651 (current as of Feb 1, 2010): "Whoever, on the high seas, commits the crime of piracy as defined by the law of nations, and is afterwards brought into or found in the United States, shall be imprisoned for life."
I'd rather see Congress expand that power to allow the use of the full force of our military power in cases of piracy and to allow military courts martial with execution at sea for pirates caught in the act. At least we do have the authority to send them to prison for life. The president (or for the next two years, the usurper) has the authority to set rules of engagement, and I'd like to see those rules include deadly force with no attempt at taking prisoners for all vessels engaged in piracy.
In the United Nations Convention on the Law of the Sea (UNCLOS) of 1982, "maritime piracy" consists of:
(a) any illegal acts of violence or detention, or any act of depredation, committed for private ends by the crew or the passengers of a private ship or a private aircraft, and directed:
(i) on the high seas, against another ship or aircraft, or against persons or property on board such ship or aircraft;
(ii) against a ship, aircraft, persons or property in a place outside the jurisdiction of any State;
(b) any act of voluntary participation in the operation of a ship or of an aircraft with knowledge of facts making it a pirate ship or aircraft;
(c) any act of inciting or of intentionally facilitating an act described in subparagraph (a) or (b).[113]
A limitation of UNCLOS Art 101 is that it confines piracy to the High Seas. As the majority of piratical acts occur within territorial waters, some pirates are able to go free as certain jurisdictions lack the resources to monitor their borders adequately.
Also, The International Maritime Bureau (IMB) defines piracy as: the act of boarding any vessel with an intent to commit theft or any other crime, and with an intent or capacity to use force in furtherance of that act.
And finally,
Hostis Humani Generis:
Perhaps the oldest of the laws of the sea is the prohibition of piracy, as the peril of being set upon by pirates, who are motivated by their own needs rather than by national allegiance, is shared by the vessels and mariners of all nations, and thus represents a crime upon all nations; as such, since the time of the Ancient Romans, pirates have been held to be individuals waging a private warfare, a private campaign of sack and pillage, against not only their victims, but against all nations, and thus, pirates hold the peculiar status of being regarded as "hostis humani generis", the enemies of mankind. Since piracy anywhere is a peril to every mariner and ship everywhere, it is held to be the universal right and the universal duty of all nations, regardless of whether their ships have been beset by the particular pirate captured, to capture, try by a regularly constituted court-martial or admiralty court (in extreme circumstances, by means of a drumhead court-martial convened by the officers of the capturing ship), and, if found guilty, to execute the pirate via means of hanging from the yard-arm of the capturing ship, an authoritative Custom of the Sea.
Should a Nation's Executive have the courage to order such, it would be nearly impossible to legally prevent high seas tribunal hearings and execution of pirates.