Well I'm not saying its wrong; you can do it in-game as well by sending a message 'to all'
I believe the rationale behind it is similar to games like
http://forum.shrapnelgames.com/showthread.php?t=43097
and other RAND games; however in this case as we know who the players are at least we can still use the board for stuff like asking for extensions + the admin can play the game too etc. I think there was another variation where all diplomacy could only be done publicly on the messageboard.