I believe that the author decided that, for some reason, it wasn't newsworthy to mention that pretty much all computers making up botnets run some variant of Microsoft Windows. Computers running Linux or even Mac OS X are far less vulnerable due to their security architecture. As directory of a IT support business (specialising in Linux systems), I have seen a few Linux machines compromised due to security vulnerabilities in network applications like the FTP server or poorly written web applications, but pretty much every MS Windows machine, desktop or server, has fallen victim to viruses, trojans, and other nasties, and many of them have (until we secured them) been part of botnets. There is no doubt that Microsoft's poor design decisions (and the fact that there are those willing to exploit them - Microsoft hasn't done much to endear itself to people after all) are the root of the problem.

Dave