I agree this is a little weird, but I can see why. These groups actually believe that what they are doing is legitimate. That they have a letterhead (or business cards...I can see it know: "Osama bin Hodgie, International Terrorist") is just an extension of this delusion.
My point is that the PLO and their minions believe their own lie: that what they are doing is the business of the Palestenian people.
Apparently, the letterhead makes them feel like they are doing their job.
But you realize what this means, don't you? Copy the letterhead, ID any techniques used within the papers to authenticate documents, and one could have some real fun with these loons.