That's not exactly accurate. Permissions are a big deal. In windows, at least before the annoying UAC popups, each account would have the equivalent of root access to everything. If that account got a virus, there was nothing stopping it from running amok. With linux and other unix derivatives, you never use root unless you need to make system wide changes, and unless you're a moron and give your everyday account superuser access, you can download all the crap you want and it will only affect that one account. Not only would your virus have to find a way onto the computer, but also find a flaw in the OS to exploit to gain access to everything else. OSX undermines this a bit by setting up the main user as an administrator, but also takes the UAC approach. However, it at least asks you for a password before making any changes. As far as market share...script kiddies and virus coders target home users, not hackers. Hackers target servers, and windows sure as hell doesn't have the majority of the market share there.
Even then, UAC is deliberately broken to allow older applications to work. Plenty of hack-arounds for it exist.
