It's an interesting dynamic with a lot of assumptions being made from those outside looking in. However, we have metrics available to us that show how much of our traffic is coming from forums. In short, the number of advertising impressions a forum gains from me posting a video is several multiples the impressions it generates on my video. Thread visitors will watch my video once, but they'll keep returning to the thread to read new posts, and every visit and every page turn on a thread is a whole new set of ad impressions for the forum but zero new views on a video.
Gun channels earn about $1.00-$1.50 per thousand views (we don't actually earn anything unless an ad is watched or clicked, though), to put things into perspective for you, but forums are likely earning more than that per ad, and there are how many ads per page on a typical forum? I stopped posting to another well-known site when they wanted to charge me 10x/month what I made from all of the views coming from all of the forums combined.
For example, the thread I started to share my video on the Ghost M currently has 1424 views, but my traffic from all non-YouTube sources combined (forums, blogs, emails, Facebook, Twitter, etc.) is currently at 749 views. Total. For all sources not directly associated with Google or YouTube.
AR15.com is some small subset of that. Who do you think made more money off my post, AR15.com or me (currently somewhere less than 75 cents for posting and then answering subsequent questions)?
No YouTuber participates in forums to make money. We do it because we like being here the same as you, but part of our ability to contribute to the discussions is by sharing videos, deals we might have for our viewers, and giveaways.
Does AR15.com have a right to charge for this? Sure. Their house, their rules. Does it make financial sense for a YouTube channel to pay what they're asking? Nowhere even close, which is why I don't know of anyone who does it.