Actually, it's not. Euthanasia is not only done chemically. Euthanasia can also be performed through cervical dislocation, as I stated before, and a myriad of other methods. When you chop the head off a chicken, that's considered humane euthanasia by the AVMA.
Slaughterhouses (in the United States at least) HAVE to abide by AVMA guidelines for dispatching animals. It doesn't matter what the end result is, being burial, cremation, or a food source. Slaughterhouses just can't use humane methods that would make the animal unfit for consumption - because that wouldn't make any sense to ruin the product you're selling.