To begin with, there are 150 domains in the top list that contain the string "virus." This illustrates the popularity and the potential profitability of distributing software that cleans (or claims to clean) infected systems.
It could be inferred then, that there are a lot of systems on the Internet that users are trying to clean and/or protect from infection. Unfortunately, looking at the domains/sites in the list, it is difficult to determine if the wares being peddled on the site are legitimate or malicious. From my experience, most legit A/V products don't include the word "virus" within their domain name. The volume and sometimes "pushy" nature of anti-virus related sites further adds to the confusion of what are real or fake/malicious.
These fake sites have been a problem for a while, but is the Alexa search result showing a rise in malicious sites? This may be a good time to make sure your website hasn't been hijacked with malware and to make sure website security is in place.