Evidentally, the US Justice Department likes obscure laws. In this case, it's a law that has been used in over 100 years -- a law against "sailor mongering".
Wired claims the law was to prevent brothels and taverns from enticing sailors to shore. Reuters says that the law was intended to prevent people from sending prostitutes laden with alcohol from going onto boats to shanghai sailors.
Whichever it was, I sincerely doubt that Greenpeace was trying to entice sailors with alcohol. What they were doing, was boarding a boat with illegal hardwood sailing to the US. They served time for boarding the boat; and then 15 months later the organization was charged with sailor mongering.
Looking at the facts, it's pretty easy to see that the US is looking to fight Greenpeace. Why, I'm not certian -- but let's face it, when you have to trout out a law used exactly twice, the last time in 1890, you've got to be pretty actively searching for something to stick.
If nothing else, this and ex parte Quirin, the 1942 decision that "supported" the enemy combatant laws, ought to encourage us to clean old laws out of our books.