some web site told them they have a virus and they need this thing to fix it
I did some investigating of one of those sites, and from what i can tell, they are using App Store affiliate links, and rotating amongst a handful of accounts. If they can convince you that you have a virus, and they take you to the $30 Symantec app that has good reviews, they get a nice commission. Symantec doesn't even have to have anything to do with those sites.