http://www.tradingcarddb.com/ViewCard.cfm/sid/4926/cid/1203900/1992-93-Upper-Deck-600-David-Vyborny?PageIndex=6
Here is an example, this is one of MANY cards of the WJC that I have in my Oiler RC PC. It is also listed as such at Beckett and in multiple publications. There must be some kind of rules as to why. I will do a quick Google and return ..........
and found this ...................
http://www.beckett.com/news/how-the-1990-91-upper-deck-world-junior-champions-subset-changed-hockey-cards-forever/
I knew I would find an answer, short answer from 90-91 to 01-02 WJC cards were allowed by the powers that be, after that the player could not be printed on a card until they had made a debut with thier NHL club (if I read it correctly). It is a good read and makes sense, not everyone will agree I would gather but at least my WJC RCs are safe. =)