Its being discussed because as horse owners, or animal owners, sometimes decisions have to be made on the spur of the moment to end an animals suffering..
Also, if a horse needs to be put down, no vet is available and nobody will shoot it, you can euthanize a horse by injecting penicillian into the vein. We have had to euthanize them that way when the owner can't bury the horse for one reason or another, they can take them to the dump and leave them there in a pit that is burned regularly. We meet them there and perform it with penicillan because if other animals eat the carcass before they burn, they will not die from the penicillan where they would die from the euthanasia formula. We try to lightly sedate them first, which will not harm other living animals, then inject the penicillan. That is why you always pull back a syringe once the needle is in the muscle, if you hit a vein, your horse will drop dead.