The main problem is the fact this is a forum, so any registered user can post whatever the like, and we tend to get spammers coming in and spamming links to malicious sites, but because these links are on this site, it gets flagged as being an attack site. We may need to get higher security in the form of anti-spam, but there's not much that can be done to be honest
EDIT: This isn't a user-submitted reporting system (at least not that I'm aware) it's a Google thing where they scan sites for known malicious links and report that to the users when they try and get on it (as far as I'm aware, it's a Firefox thing only, since I don't get it on Chrome, which is weird because I believe it's run by Google)