I think it is a gut thing, which I completely understand. As I think Bentley said, you will probably only completely get it if you run forums yourself.
I run a couple of forums, although no BBQ ones. My main one, which I have mentioned before, is for the local board game group, which is a combination of social, information and scheduling game sessions.
People often link to BoardGameGeek, which is probably the biggest board games forum going, but it has information on practically every game available. So, in context, someone might be saying "Let's get together to play this game; if you don't know it, here is some information on it". I have absolutely no problem with that.
But when people try to link to other clubs running events in the next city, or to the generic Meetup.com site, I get very irritated, and tend to clamp down on it. BoardGameGeek isn't taking discussion away from my site, those sites are.
Similarly, there was intense pressure a few years ago to have an associated Facebook page, and a couple of people started one, until I protested (they were within their rights to start a page, but not with my website name!). I have seen a couple of forums that started a FB page that ended up taking all the traffic from the website - notably the British BBQ Society forum, which now has about 3 posts or less a week.
In the end, I created my own FB Group for my site, but that meant I could control it. Every time a game session is posted to the website, a post appears on the Group page, but just with the Session date and Subject, and a link back to the website for discussion. This seems to have kept everybody happy - the Facebookers get told whenever something is scheduled, and can also chat and post pictures of their games in action on FB. Meanwhile the main action - scheduling sessions - and meatier discussion (on rules and game variants) remains on the website, which pleases those who aren't into Social Networks. Every now and then a newcomer will try to tip the balance towards Facebook, and need to be gently corrected. I know they mean well, so I lean on them politely.
Hope this makes sense. In the end, it's your site, so your rules. I think if you make those rules nice and clear, no-one can complain.