Sites get tagged mostly because their ad servers have no scruples whatsoever about running ads containing malware. The adservers protest this vemhemently, but seem unable to detect such malware ads, though even freeware antivirus software has no problem detecting them. They might want to switch their domain, or at least switch their ad server. INE (In my Experience) Adgardener is one of the worst offenders.
So why doesn’t Weasel Zippers get rid of those ads? Right now those warnings must be killing his traffic.