on the face of it I still can't understand what they could be thinking

I suspect they're thinking "instead of writing these documents in a way that only some of our readers can understand, why don't we try to write them in a way that all of our readers can understand." There's quibble-space on some of the terms excluded, but the basic principle seems sound to me.

And the Plain English spokesperson's e.g./egg confusion might be silly, but there's plenty of people who don't know the difference between 'e.g.' and 'i.e.'