It’s just link building Ballss. This is what that SEO company does when you pay for for good search results on google. They just pay spammers to find places like this and go for it and do it in such volume across the web that google sees the site better with whatever they have for their criteria. They hope for a board with a ton of activity so it just gets pushed off the front page but stays on the site, and it works. For every site that goes and deletes them, there are others that don’t.