My experience comes from a different (IRL) context, but the type of people you need to moderate or ban are also often the type of people who will move right up to the edge of any bright line you draw, and use it is a shield for further antisocial behavior. Sometimes ambiguity is its own reward, especially when the lines are well-understood by most already.
As just one example I'm the resident "soldier" for the government (as one HN user was so nice to call me), I often post comments that are in opposition to many of the tenets that are popular on HN, and yet I've not been banned, my karma is not negative, and those I debate with generally treat me and my arguments with respect. If it were just up to ideology I should have been hellbanned by now, and yet here I am.
As just one example I'm the resident "soldier" for the government (as one HN user was so nice to call me), I often post comments that are in opposition to many of the tenets that are popular on HN, and yet I've not been banned, my karma is not negative, and those I debate with generally treat me and my arguments with respect. If it were just up to ideology I should have been hellbanned by now, and yet here I am.