Well in Google's case it's easy, wouldn't want to upset that ad income. Firefox though absolutely should be blocking/warning of malware for sites like this. Firefox is more spin than substance these days, sadly.
But these are companies overwriting Google's ads with their own. Sounds like Google is directly hurt by them. Maybe blacklisting would get them charged with anticompetitive behaviour? (That would be really funny in a horrifying way if that happened.)
