It did happen in Maryland. Guess what? The CPS completely changed their rules after the justified outcry.[1] Not a single person was arrested in this matter.
Because this happens every once in a while it’s very easy to get the idea it’s an official policy and you can get in trouble for it. And the consequences are so high people don’t want to risk it.
Add that to internal fear about stranger danger and the kid getting lost or hurt and is it any wonder people tend towards not allowing this stuff?
Remember, any time the police are called, even if it's stupid, they have to show up and investigate, and when there's no parents around, that means they have to find them, and at the minimum you have a giant hassle on your hands.
Even in the incident you cite, the CPS did act badly, and that was just in one place. Why did they ever get to the point where they acted like that in the first place? It would have been unthinkable in other developed nations.
And in your own link, the last line is: "The encounters between the Meitivs and the government have led to an intense debate among parents and educators.["
Why is there an "intense debate" about this? Obviously it's a big, big issue in the US, or else there wouldn't be any "intense debate" about it.