Best answer by explanoit
In addition, in consumer mode you would never be able to install any applications, especially not something as low level as antivirus. The operating system is mounted as read only.
You can read the technical overview of these features and design desicions on this page. http://www.chromium.org/chromium-os/chromiumos-design-docs/security-overview
Although recommended on normal consumer machines with fairly permissive user abilities, antivirus is not a universal requirement for secure systems. But your instinct of wanting antivirus protection on your machines is not a bad thing to have.