There's also this little thing called word of mouth. While many customers may never visit the forums, many of those customers may have been brought to 3d because of a friend. That friend might visit the forums. Or it could be a friend of a friend of a friend. They could belong to Facebook, Google+, deviantart, or some other non forum art type site, and in their connections and explorations there, hear things. It's too much of a social media world now NOT to believe that just because a person never visits a store's forums doesn't mean they don't hear about things that could cost a store their business.
When people are upset at the actions of a store, they don't just post about it at a store's forum. They bring up their discontent everywhere. At work, at home, among friends, on their cell in elevators, every social media they use. Conversations are overheard ... and while most of the time those people won't have any clue what the discussion is about, it's always possible someone will. That person will then spread that little bit of gossip they overheard. Even if they have no clue, they still may spread that little bit of gossip, and someone they share it with just might be a customer.
That's my 2 cents worth anyway