Actually supplying security updates even for "illegal versions" makes sense since as the article rightly points out a virus is a virus and if it is spread to "legit users" then its bad publicity for the OS as a whole -- and in anycase since the code has been developed it is probably easier just to allow everyone to update instead of "irritating" users with extra nagware (such as WGA) so much that they just stop bothering to apply the security patches and help to spread future viruses.
Not so difficult to understand this philosophy actually.
Cheers
jimbo