When is a drive by OK?

I've linked to this post before—The Funny Little Universe at Tied to the Tracks. I've already posted an except from her description of receiving a drive by, but earlier in the same post, she talked about a situation where she wished she had spoken up and didn't. I've been in similar situations (although none quite so awful), and I sympathize with her dilema. Do you speak up? If you do, will it help? How do you distinguish between a drive by and necessary intervention—especially when some people are willing to claim, for example, that allowing a baby to cry it out is a form of abuse?

Here is an excerpt from her story . . .

I agree that parental drive-bys are the ultimate in poor manners, and I try to keep my opinions to myself. And yet, I draw the line at public child abuse, and will, in cases where a child is being abused, speak up. I have done this only twice in my life, and both times were highly traumatic for all parties involved, but they aren't the situations that come back to haunt me. What I think about a lot is the time I did not speak up, and should have.

When she was six, my daughter broke her wrist jumping off a tree stump at day camp. We ended up in pediatric orthopedic care at the University of Michigan's hospital so she could be xrayed. We were just hanging out there in the xray suite waiting our turn to talk to the doctor before we went to the cast room. There were three examination tables in this particular room, one empty, one ours, and on the third one, next to us, a little girl maybe ten years old. She had an elaborate cast on her left arm, the kind that has a metal bar to hold it in a particular position. There were xrays on the wall light box and I could see she had three pins in the bone of her upper arm. This was, in other words, a damn serious break.

Her father was with her. A guy maybe thirty five. Well dressed, middle class. And hissing at her like a snake. I can call it up with perfect clarity all these years later. It went like this: don't you cry don't you dare cry you baby you sniveling baby you can't get away with that with me maybe your mother puts up with it the bitch but not with me. shut up shut up shut up. And it never stopped for the ten minutes we were in that room together.

The girl was weeping, tears running down her face in a steady stream, her whole body shaking. And I said nothing. Why? How could I not tell him to SHUT THE FUCK UP and leave the kid alone? I wanted to.