Because they do not know what kind of animal ferrets are.
sponges and ferrets
Ferrets do not turn on people if properly cared for and socialized. An abused ferret would become a fear biter. A ferret that is sick or injured (as any other animal would) might bite.
Ferrets do not yap. Ferrets can squeak, and ferrets can dook. Dooking usually means a happy ferret, squeaking or squealing can mean pain, fear, or all manner of things.
A ferret's defense would be biting - as in being a fear biter.
Ferrets are owned by people all over the world
No ferrets are not hunted, they are a non-game animal
Ferrets can help people by giving them a good laugh and can be your best friends. They can keep you company if you get lonely or depressed.
There is only one known breed of ferret that is not domesticated. This is the Black Footed Ferret, which resides in California. This species is the reason that domesticated ferrets are banned in California - people fear that if the domestics escape and breed with the Black Footeds, they will "taint" the population of this endangered species.
Baby ferrets need to be socialized by humans, like any other domestic animal, and in doing so will crave human attention. If ferrets are not socialized, they will fear humans.
Ferrets don't attack. Improperly raised and abused ferrets can become fear biters, just like any animal.
Ferrets are not illegal in all states, or cities. Every state has their own rules on owning ferrets. There are only 3 states out of 50 that ban ferrets in America
Ferrets are legal to own as companion animals because some people prefer to own ferrets for pets as apposed to a dog or cat