I can't speak for the mods/site owners, but when it comes to website development, search engine optimization and page ranking is the key to attract traffic to your site and locking your site off is a great way to kill off search engine web traffic.
You'll notice CGN locks all their forums EXCEPT vendor forums. You'll notice CGN's ribbon bar has an enormous amount of sponsors and I'm pretty sure they still have a user base in the 100,000 range - ergo, they can probably (for now) get away with locking down their forums for non-registered users. They have a huge base of users and revenue sources and not much need to aggressively grow their forum.
If the GOC founders/owners aren't locking off their forums it's because they are actively trying to grow their site - which I think is a good thing.
So as an example, if I'm a gun loving brony googles "AR 15" "My Little Pony" and sees 10 threads started on GOC by a guy named Harbl_the_Cat, including thesis length analysis of individual episodes with philosophical analysis of each different pony and how each of them personify a particular technical feature of an AR 15, they are inclined to visit the page.
If they can read through Harbl the Cat's detailed and insightful posts and from each one they find a thorough comparison about the ballistic characteristics and terminal performance of apple pies versus party cannons, they are inclined to join in on the discussion, further contributing to increased page rank, greater short and long teal search engine hits, and more traffic drawn in, along with registrations.
In other words - more people need to write mega posts, at least 2000 words long, so that if someone does a search on Google for a related topic, GOC will be page ranked higher. I suspect that's one reason the mods only give me the occasional light flogging about posting random BS in huge quantity.
Case in point. As of today, Google search: "Canada Guns Bible study" guess what's page ranked second?
That's right:
http://www.gunownersofcanada.ca/showthread.php?13569-Bible-Study
Perhaps Jwire and Zulu have me on payroll as a GOC shill...
Seriously though - I don't think the sight owners should close off the forums - I don't think most of the members should want them to either.
Remember, the Tavor contest REQUIRES 12,000 members to join by December. GOC's at about 8500 and the year's half over - so there's quite a way to go...