I don't know whether any of the adults couldn't swim. Nor do we know from the news how long the group tried to rescue the boy before they sent someone for help, nor where the adults were at the time of the accident.
2. We know they did not try very hard to rescue the kid. If the river was not that bad where they were, they would have not have lost the boy. If the river was really really bad, they would have lost a scout leader as well.
The adults were supposed to be "with the boys"- that is the whole point of having scout leaders. If the adults were not around, they should not have taken the kids from the safety of their homes.
Would you let your young child be taken away to a raging river by someone who cannot swim, who will not be nearby nor will be watching him, and who will not jump in afer your child if he should fall in?