Exactly, forums even if they were 15% of the game's population are still not representative. It's not just the size of the sample that matters but how well it reflects the overall population. Forumers, in any game, will always people who are a bit more passionate/obsessed/annoyed.
Those who log in for a few hours a week will never come to the forum because they aren't that interested. Despite their lack of interest BW/EA still want their subscription money so they have to care about them.