So you have a chicken having its throat slit for a religious ritual. Blood squirting out and it running around if it gets loose. Not passing judgment on anyone but can't you see why some would think this is not humane?
People get hung up on the religious ritual thing. What's the difference if they are slaughtered in a slaughterhouse for food, or first take a trip to a religious backyard to be slaughtered? They both end up in someone's tummy.
Saying religious ritual makes it sound like if not for our religion, these chickens would still be alive. I'd venture to say that Kapparos may not cause even a single extra chicken to die - if they weren't killed there they would be killed elsewhere.