Sure, I don't fault Facebook either. I think they're wrong, and making a mistake, but I understand why, and think it's a mistake they are absolutely allowed to make.
Technically, Armslist is legal. They don't even facilitate transactions on Armslist, and it's local laws that apply. Basically, it's a Craiglist for firearms. Armslist has also won numerous lawsuits[1] by claiming Section 230 protection. Facebook doesn't know this, and they have no reason to know this. Acquiring this information about Armslist would require someone up high enough at the company to impact US policy to understand this. That would cost money. It makes more sense to avoid it altogether, just like it makes sense to avoid pornography altogether, rather than watching every video that gets posted, in case a "model" is underage.
Matrix gives me the perfect opportunity to continue this conversation on my own terms, using my own knowledge to moderate it.
However, there's also another reason that inspired me to ditch Facebook, which is the fact that they are censoring my private messages at all, without any action by a human being to report them or anything like that. I just dislike that policy, and so I chose to stop using Facebook over it.
[1] https://www.armslist.com/blog/get-involved/armslist-defeats-...