I hve been looking through the rulebook but can't seem to find the impenetrable terrain LOS rules.
Do you have bkc or bkcII?
But what i think is going on is a standardization of impenetrable terrain. Impenetrable terrain can be clifs but can equally be a swamp. The first you cannot look through and the second one you can easily look over.
Maybe Pete made a compromis for including situations like that?