In all the times that I've attended the Scout's Youth Protection Training, I always heard that we shouldn't be alone with a scout. It never occurred to me that I shouldn't be alone with another parent. Sheesh.
Obviously, it's a matter of judgment, rather than a BSA policy. However, the point is the same: that if the situation (adults alone, or adult alone with child) simply doesn't happen, then there can't be any problem. Not abuse or assault, not honest misunderstanding, not deliberate deception.