Zen chose them for a reason. She sees things we dont and from this has a better understanding of each moderators abilities and temperament. If a player has an issue with a mod those concerns can always be taken to the admin for mediation. Basically if a mod isnt doing his/her job that mod more then likely would be replaced.
I also dont see a need for a SOP to be public. I hope that there is a standard, but any standard made would be set between admin and mod and not really up for public debate. If such a thing were to be public I am sure there would be many out there pointing fingers and jumping all over the mods the moment they make the slightest mistake or deviation from that SOP which would only undermine their authority. Just my opinion.