Doesnt take much to modify the viruses to attack other systems.. But why bother? Doing it for Windows ensures fast and wide spreading due to the massive user and server base. Their goal is to see networks shut down, and they often get the results they desire. You could probably make the same type of virus for the MAC. But when they make up less than 1% of web servers, and support no major backbones or networks, who is going to care?