Excellent point. Even the very best AV software only detects 99% of the known viruses (meaning >100,000 viruses cannot be detected, although the odds of seeing many are near-zero)) and any new zero-day viruses (they use abnormality detection algorithms for these).No machine is ever %100 virus free. They sell you the false sense of security. Any top notch skilled hacker can hack your chromebook if he/she wanted to added that if your a DFU and doesn't use any common sense.
http://www.mobilegeeks.com/chromebook-virus/
People keep talking about Linux (Apple OS is based on a Linux Kernel) being all but invulnerable, but nothing is further from the truth. More and more AV vendors are realising products for Linux builds for this very reason.