This is specifically talking about mitigation for highly pathogenic avian influenza. HPAI kills chickens fairly quickly, so to contain the spread and minimize the risk of zoonotic spread to people, they kill every bird on every property that it’s detected on.
This is one of those situations where no one thinks it’s a great solution, it’s just a pragmatic one that minimizes the risk towards workers while quickly depopulating the barn. The problem is that this is one of the cheapest and least humane ways to depopulate a barn, and shouldn’t be allowed. We should insist that barns allow humane depopulation, or at least less inhumane methods.