I don't think that walking to the local shop is the same level of risk. When the grown ups are asleep in their house, they are leaving the kids unsupervised for hours on end. Different if they were in the tent next door, but I do think if they're indoors it, to me, would be a risk I wouldn't take.
That said, I wouldn't judge parents for allowing it or say children shouldn't do it. It is up to parents to evaluate the level of risk which is specific to their house, area, age of children etc.
Also, as has already been said, it isn't just the unlikely chance of a child killer being in your particular area at a given time, although, that risk is there and there are predatory "local weirdos" who do take an interest in children and if they are given the opportunity, they may take it. But it is also the lower risk things, which I still think, for me, would outweigh the benefits of letting them camp out unsupervised.
I used to be a Scout leader and I remember taking a group of boys about this age away for the weekend. One of them got homesick and a bit overwhelmed and we heard him getting out of his tent in the middle of the night, in the middle of nowhere, and wandering off. I don't know if he was sleepwalking or what, but anyway, we obviously spoke to him and made sure he was ok and got him back to his tent. If we hadn't been there, he might have wandered off into the woods and got lost. I know people will say "there are no woods in most people's gardens", but I still wouldn't want mine wandering off, even in our pretty safe area, in the middle of the night.
Some people will think differently and that's fine, but I don't think there is any "this is what your dcs must do or they will surely become emotionally broken by age 19". It is one of the many decisions parents have to make and I really don't think either view is invalid.
I do think some people on MN seem to think life is some Enid Blyton novel and I wonder if I live on another, much less sheltered, planet sometimes.