Just a thought but maybe they dont have rain or fog on the maps because of the interior sections? I cant remember ever having weather effects during a siege in vanilla M&B and every other scenario only had at most buildings that you couldnt enter. So with maps like village and the siege maps which do have interior parts this would probably be a problem, unless they have zoning or something to tell the game where to apply weather effects.
The only maps I remember getting weather on were the randomly generated open plains maps, no buildings, no interior sections.