Cause it's a community oriented forum with no central authority. The users are allowed to give their suggestions on what could be or not be a scam, offering their evidence to that effect; Everyone else is allowed to make up their mind using the suggestions of others as pointers.
Some things are not moderated like scam, trust system, merit system but there are many things that are moderated because forum have admin, staff and moderator who carry out their respective function. Do they deserve to be called a central authority where they can do several things for the forum community such as moving thread, deleting post, merging posts to ban users? If I misunderstood the central authority you mean, please clarify.