Ah, that's right, he didn't leave until after the group had returned. Though he still had quite a lot of time alone with this adult. Odd that he waited for the group to return, if he'd decided to take off. Maybe the adult WAS keeping a very close eye on him, and then once everyone was back it didn't seem so likely that he'd leave, so he was no longer being watched so closely and grabbed the opportunity to make his run.
I get the part about breaking the "never alone with a kid" rule out of practicality -- I'm not sure how many other adults were on the trip, but perhaps not enough that leaving two behind with this kid would enable sufficient supervision of the hikers. BUT if the situation with the kid was so bad that they felt they had to break that rule, they definitely should have called the kids parents to alert them to what was going on, and try to arrange to have them come pick him up nearby. There's no indication that that happened. When it's no longer possible to supervise a kid within the rules, the very least that needs to be done is to notify the parents, and in the age of ubiquitous cell phones there's no obstacle to doing that while on a camping trip.
It does sound like this kid isn't quite all there, and that the parents and scout leaders may not have been facing up to the degree of that problem -- you just can't make things okay with a mentally disabled kid by pretending he's normal and having him participate in everything the other kids do as if he was normal. Twelve is awfully old to pull a stunt like this without some major extenuating circumstance like being molested. My gut feeling is that this kid is seriously troubled for some unknown reason, in addition to not being of quite normal intelligence.
I find it very interesting that there has been no wall to wall coverage of this from the moment he was back...You make excellant points and perhaps it is as simple as a lot of adults not aware of how dangerous it can be for a child who is "not quite with it" on whatever level.
We are now sooooo politically correct that problem kids are not getting the "seperate and more intesne " coverage they need.