Here is what the AKC website has to say about the breed standard:
Keen intelligence, an independent spirit and innate dignity give the Chow an aura of aloofness. It is a Chow's nature to be reserved and discerning with strangers. Displays of aggression or timidity are unacceptable. Because of its deep set eyes the Chow has limited peripheral vision and is best approached within the scope of that vision."
No one is saying they are vicious, just not easilly accepting of strangers.