Given these responses, I decided to conduct a very unscientific survey, and ask some distributors ("horses' mouths") why they do what they do. I only got a hold of a handful of people (from the East and West Coasts), and the general reply was the following: The brew pressure is set high because they want to avoid the "Oh my, I have a defective machine" call. The problem tends to be that a new owner will grind too fine and tamp too hard and wind up choking a machine. The brew pressure is set higher than optimal so that one really has to try hard to ring with a technical problem. Well, that was the general response. There was one other response, namely, that the higher brew pressure is set to get an actual dynamic appropriate brew pressure, but that depends upon the machine.
In short, I got both responses given above, but with a predominance of Ian's account, though one basically agreeing with Jim.
Still, a question: Does the first reasoning take into consideration the findings of a higher pressure associated with lower flow rate (higher pressure compacting cake, reducing flow rate)? Further, since the brew pressure is set as a maximum BP, it would impact ristrettos (or the inadvertent ristretto--overfine grid and tamp), and is not such an issue for normales--so it would seem.