Most forums require login for posting, but for reading does not. If reading requires login, it is breaking TOS, because the Media robot can not access and determine content of the pages. I think big servers like newspapers or really big forums requiring login can make a deal with Google if they allow their bot access password protected pages so the robot can read thme like members can.
For example the platinum section here does not have Adsense ad and I think is because of the TOS...