You are right, when I rewrote the SitePoint Forums Guidelines, I used several large community sites as my basis.
These included iVillage, Thirdage.com, AncientSites.com and MotleyFool.com.
I got this list of sites from the book "Community Building on the Web" by Amy Jo Kim. This book should be on every community builder's desk.
Anyway, our guidelines have gone through several versions over the life of the community to get where they are today. Our first version was fairly incomplete and caused us to react for every issue.
This lead to a new version that continued to get more and more complex as added rule after rule. Eventually when we had 34 different rules for banning people and we were restricting the movement of our members, I knew something had to be done.
This is when I rewrote the guidelines that we have now. Before this version we had rules, now we have guidelines. There is a subtle difference. One I am not going to ban someone for ignoring our signature rules unless they blatantly refuse to make changes at the request from an Advisor.
Also if you look at them they outline different things that are encouraged and discouraged. We want our members to feel comfortable and participate while protecting our right to maintain control as needed.
Where the format originated, I don't know. It seems to be one of those silent standards that take over because they work...
A lot of the large community sites online today started as AOL Greenhouse projects. These were started in the early 90's to give AOL access to content. In return for exclusive access to the content at the time, AOL would finance the new sites with $25,000 in capital.
Motley Fool was one of the first recipients of this capital infusion. iVillage, Babycenter.com and others started this way as well. It could be that the format of the community guidelines echos the ones used at AOL in those days.