In essence, they're telling their employees they're too stubborn or stupid to follow guidelines and use their judgment when it comes to responding to email.
My point was that the average employee in a large corporation doesn't see the implications of a reply-to-all, and how much e-mail overload and lost time it amounts to down the road. This isn't implying that these employees are less intelligent or otherwise impaired, it is simply implying that this is not something an average employee would think about. Just like I will forget to change the oil in my car: Obvious to a mechanic, but I just don't think about it, or don't know.