This was explicitly implemented because of the spammers (quite some time back in fact). I know for a fact we probably still have somewhere in the order of 300 or so fake spammer registrations on the board prior to implementing the two changes we put in place to combat this.

There is a specific mechanism where when people register on the forums, notifying them that they need to post an introduction before they gain access to the rest of the forum. As soon as they make that first post saying who they are and hello, they automatically get promoted from "registered" to "registered and introduced" which grants them access to the rest of the forum. Un-registerd people can only see (and post) in the guest forum. The guest forum is there for people so that they can post if they are having problems.

I have to check as I don't remember, but the registration confirmation email contains the instructions about needing to post an introduction, and also once they have registered and logged in they also get a pop-up PM notification as well.