I think it has something to do with the redirectURL. Whenever you post direct links on the forum and someone clicks on the link, there is a redirectURL that is being forwarded to the 4shared that from which site the user has navigated. That way, they can set up a parameter to automatically block links which has this forum's redirectURL.
In my opinion, it would be good to only paste links like /folder/ABCXYZ or /photo/ABCXYZ or /video/ABCXYZ. Maybe we'd be able to keep the links alive for a little longer. This way, they will have to scroll the forum or each post in order to block the links.