Why is killing the dog necessary, rather than just spay/neuter and then letting it do something other than herd sheep?

The dog would need to leave the farm, and couldn’t go to another farm or a home with children. That mostly leaves rescues as the option, and they often don’t have much space, and the farmer or rancher may not be able to safely pen the dog until such space can be found or may not be able to afford feeding more dogs than needed to operate.

*But its the only way."

Are these dogs unsuitable as pets (for non-farming households)?

Sometimes. Border collies will happily herd your children if no sheep are available. You wouldn't want one that had bitten a sheep.

>Are these dogs unsuitable as pets (for non-farming households)?

Does the household have any toddlers to herd?

Working dogs that are not elderly to the point where they prefer to sit on their butts all day generally make terrible pets. Ones that are bred to herd animals and have been kicked out for biting them even more so.

I think it depends if you have the time and energy to exercise them. Kelpies, Border Collies and Australian Cattle Dogs are popular pets here in Australia, you just need to exercise them.

They are very intelligent, alert and loyal. We adopted a Cattle Dog and just had to be careful around toddlers due to herding, but otherwise he's very well adjusted.

He will follow you up the stairs and nip at your heels as you go, but he never bites hard and does it for attention/affection.