This is usually triggered by someone with the same or a similar IP address (eg: same provider) being infected
Eg: If JJJ using IP is infected and your IP address is aaa.bbb.yyy.zzz then you might be presumed to be infected.

While you are directed to a site that offers a purchasable version, there is a Download link directly underneath it (I just opened the site) where you can download a free version.

It would help to determine if this is literally coming from the server, or if someone is trying to spam/force you to purchase from that site, if you could provide an exact quote of the message, including the nick/name of the sender.

Even server notices contain this information.