LinkedIn actually has a place where groups can specify their "rules" but often this includes an initial statement of purpose. For SMT, for example, it states: "Our intention at Social Media Today is to host a group on LinkedIn that fosters conversation and exchange. We are not an advertising or promotional platform and - to the extent that we can - we will filter out such submissions."
My feeling is always that clear and transparent rules are essential to good moderation. I moderated a very explosive FB group once and I made it a practice of explaining to the community anytime that I deleted comments or blocked a user. I actually found that the response was overall fairly positive.
Of course in that case it was dealing mostly with trolls and heated, personal attacks on other users but I was very clear from the start that the group was dedicated to proactive discussion and listed what would be deleted/moderated out.
Most good quality groups and sites have these policies in place. If you want to see examples of a lack of moderation you need go no further than the comments section of any online newspaper.