I certainly wandered around the neighborhood when I was 9. That was sort of the point of living in the burbs - it was safe! And, no, stranger danger isn't a bigger thing now than it used to be. I also got dropped off at the pool..maybe not at 9, but certainly at 10 or 11.
Even if we truly believe this is a child welfare concern, "arresting mom" is clearly a bigger concern than "left at park." Not exactly a mystery what will be more scarring.