the line is as thick as society's range of opinion

I'm not surprised. I had the "don't talk to strangers" paradigm pushed down my throat alot during childhood. From my experience, adults didn't pay attention to me and my peers other than in supervisory roles; I don't recall ever being talked to by a stranger who wasn't a teacher or peer's parent.

The paradigm is basically to treat strangers as potentially being the scum of the earth, which not surprisingly leads to rudeness and lack of friendliness, and it would not surprise me if those social habits carried over to adulthood.

While having a child not talk to strangers (assuming they actually don't) may be effective at preventing some of them from being abducted, there's certainly more sensible social habits that could be instilled in them instead, such as not accepting rides from strangers. Having a 'code phrase' for parent-okayed adults that the child doesn't know could help make this more practical (for rotary car-pooling or something.)

I hope we can agree that the percentage of adults trying to abduct children is low enough that it's not sensible for children to act coldly to all strangers, and that as a whole, the losses from such coldness outweigh the losses from the small number of abductions (from strangers) that occur each year. Ask any parent in grief over the loss of their child and they'll proscribe draconian measures against all people and situations that may be responsible. These aren't the people we should be listening to for sound precautionary advice or sensible punishments. Combine in-grief parents with sensationalist media, throw in elected populist representatives, and you get the current situation of laws created in knee-jerk response to tragic but rare occurances.

What it boils down to is that the adults aren't thinking about the possible negative effects of their (perceived to be) well-intentioned actions.