I guess it all depends on what part of the country you are from. I'm originally from California, where all you hear are tragic stories of kids getting kidnapped or raped. I certainly would not let my child go to a public bathroom alone at that age. I remember several years back, a 12 year old boy was murdered in a public restroom. His aunt walked him to the door and let him go in alone. She had no idea that there was some maniac in there who was waiting for a victim so he could slit their throat. I think this happened in San Diego. Now, I know that the chances of that happening are slim, but that story was so horrific that I know I wouldn't send my kid in a restroom alone unless I had no other choice.
It seems like the worst stories come out of California. I can tell you about a 5 year old that was kidnapped, raped and murdered. When she was kidnapped, she was playing in her own front yard. I could go on and on with these type of stories. There are some sick and twisted people out there.
Now that we live in Kansas, I see little kids playing outside and walking home from school all alone. It's rare that you hear stories of kidnappings, etc around here, so parents don't give it a second thought. But I certainly wouldn't pounce on that parent and lecture them. It's their kid and their choice.
Now to answer your question as to when I was a victim of a drive-by parent. I don't think I ever have been approached like that. That parent had a lot of nerve to approach the op. Like I said, it's your kid and your choice.