I've seen instances of infected computers spreading their disease, and the owner was alerted but they failed to do anything about it. They just didn't care. Should they share some of the liability?
They should, but it cannot be solved by writing up the law that would criminalize millions users. This could be handled by entering some form of a contract between an ISP and a user, in which case there needs to be made a regulation for Service Providers - who have enough expertise and resources to address the issue - rather then individual users, many of whom are totally unprepared to deal with these issues.