I think most people don't want to get involved, and many don't think it their place to do so; live and let live, even if that means ignoring someone else who refuses to do so. I'm not sure that's especially American, but the mystique of "your right to swing your arm... " probably makes it more pronounced, makes us more libertarian. Perversely that can create a herd mentality whereby once someone ELSE steps up to say, "NO!" it's suddenly OK to follow suit, as if someone else taking the initiative represents societys permission to do what they knew they should've already done. Of course, it's always tempting to pile on, too; that's how lynchings led by half a man work.

As long as it were strictly verbal I'd probably just keep my thoughts to myself; believe it or not, I usually do away from the internet. Babies left to die in cars are another matter entirely, and if I weren't sure the cops I called would be there soon I'd probably start looking for a rock. While it's never come up, I like to think I wouldn't let three people attack one.