One important point here is that while the guy was a scout leader, the abuse happened in his house with friends of the kids living in his house, NOT with other boy scouts.
The BSA has a strict rule that no leader can be alone with a scout, to help prevent this type of abuse.
In fact, before any adult can go on a Scout Function, they must attend Youth Protection Training, which goes into detail all of the "Do's and Don'ts"
Of course somebody who really wants to harm the kids will know that it's wrong. But it certainly is how the BSA covers themselves legally. But even with all of that. A parent still must be vigilant just the same.