Evidence for many (maybe most) cases is gathered at the interrogation stage. The larger the group the likelier it is that an investigation involving talking to the people there will uncover everything.
Not only that, you would be amazed at how many people who have done something wrong end up with a guilty conscience and just tell the truth. Especially if there was something wrong going on here and the FBI hangs the "first to tell me what happened won't be prosecuted".
Err, the other obvious explanation is that whoever was at these meeting really don't have anything to hide, and Arrington is grossly exaggerating the few anonymous tips he's gotten.
Cops can be good at their jobs.