The only way they could do it that would be 'humane' would be by shooting them in the head, and I'm sure you can imagine the safety issues, the possibility for injuring rather than killing, AND the public outcry over such a thing. Plus, the trauma to the human doing that. Shelter workers do have increased suicide risks as is. I recently had to perform 7 euthanasias in a single day at my shelter (all privately owned animals) and that was with our nice, gentle drugs. If I had to regularly do that many I'd quit the field entirely. I certainly couldn't shoot them in the head without probably turning that gun on myself.