I would suspect that the fact THR is rather tightly moderated to keep it restricted to firearms and relevant political/civil liberty topics helps keep the activity and bandwidth usage to a reasonable level. (or at least not breaking the "shoestring") And that in turn helps keep a cap on the costs. Also note that THR does not support avatars, the automadted "post with quote", and has a limited number of other graphics and emoticons or "smileys".
If you look at other gun forums that have "general discussion" and "fluff" sub-forums, you'll often find these contain thread counts and posting activity greater than any other three or four of the firearms/political sub forums combined. And these sites are often advertising, donation supported, or both, to cover the regular need for server costs, bandwidth, and general expansion.
There is nothing wrong with that. Humans naturally want to socialize and chat. Only the most maniacal of us could talk firearms and politics 24/7. However, the socializing can detract from the "serious topics" that were initially the reason for your discussion forum. One way isn't better than the other. It is simplay a matter of what the site owner's priorities are.
It is like comparing a Formula 1 racecar and a Hummer. Both are "cars", have engines, wheels, and drivers, but beyond that, they serve very different purposes. As such, so are the different Internet "gun forums".
As a result of the way it's moderated, THR is one of the most well behaved and "on topic" boards I think I've ever seen.
This of course, is just my personal theory of how and why THR is run the way it is, so someone "in the know" please correct me if I am wrong.