if it was, its not anymore. I just checked.
(and you can too, just throw your browser into incognito mode, come to the forums, and you won't be auto-logged in.)
From there, you can login, or "register"... Click Register, and it takes you through the normal webpage sign up BS...
Do note that the username "Guest" is already used by anyone who comes to the forums... so if they wanted their registered name to be Guest... well...
Or, an other option could be a geo-block. Either by the country of origin, or by the webpage against a region.
AS to why? Well, lets just say some countries
really disapprove of Shadowrun and elements in it, and leave it at that... Or the Mods have noticed that the majority of spam comes from a region, and no other (active) users, So why not block the region that is flooding you with spam? (Seen this thing before. Not here mind you)