[Adastrea]

I have found, from my intrests in history, that it seems My Lord was an acceptable term to use to address a King (though most would have gone through the motions of courtly etiquette first)and that by naming Legolas as son of Thranduil clearly shows him as a prince, no need to say the words if you clearly state he is the son of a king.
Another possibility is this: Tolkien's work seems in the most part more concerned with the High Elves, maybe they did not recognise Thranduil's claim on the title???

Also in the Unfinished tales, does it not state the possibility of Amroth being the son of Galadriel and Celeborn?

also to go quite far back in the topic, I think it more than likely prince William will one day be king, he is only second in the line of succesion. We brits don't plan on getting rid of our monarchy any time soon, no matter what the media say, after all without them who would they have to dish the dirt on?