Actually sod it I might as well -
AIBU's moderation is probably directly related to its advertising business model
AIBU is probably the one part of the Mumsnet site that gets the most eyeballs.
For those who post, it's fun to challenge the views of others.
For those who lurk or watch, it's perhaps a spectator sport seeing the insults and verbiage flying back and forth.
Eyeballs on posts directly translates to advertising revenue. Hence why you get all the pop up videos on AIBU, the inline ads etc. People want to pay money to get in front of the vipers.
And a lot of us vipers (according to the numerous salary threads) are ABC1 to use the old social grading, and many are A and B.
A heavily moderated site would remove the fun for a lot of people. No more "handing someone's arse to them on a plate". No more tearing down an OP's premise or ripping apart their spelling.
That would lead to fewer eyeballs on posts, people will go elsewhere. Which won't do for ad revenue.
In terms of regular posters, I would wager they get an easier ride too to some degree.
If 40% of your posts come from 10% of your users, then you want those people kept onside because they keep the debate going.