Although it's true that nothing put online is truly private, and people should know this, I think there's a double-standard going on.
If someone vulnerable posts about a personal crisis here, saying they have no real life support and they need to talk, MN users will generally respond as if they have a duty of care. Not in the sense they're individually responsible for helping this person, but any replies that are dismissive, contemptuous, cruel or focus on some minor point (poor grammar, lack of paragraphs) will be loudly criticised and, depending on exact content, reported or deleted.
But it's not illegal to respond to a desperate post about DV or gaslighting by saying it's 'I should HAVE left him, not I should OF left him, come on' or you seem like hard work. It's just extremely unsupportive and wankerish. The fact that it's considered unacceptable on MN suggests that this is a community and not a random free-for-all bulletin board or gladiatorial arena where the only restrictions are libel laws.
Yes, the DM can legally skim off whatever they want, regardless of personal hurt caused -- not just to the OP, but to all MN users: the upsurge in deliberately provocative or divisive posts designed to get in the paper; the constant speculation about who is a journalist; the people who simply don't post because they can't afford to have a sensitive situation broadcast.
Just because the DM has so far stuck to threads about social etiquette or sex, doesn't mean they can't migrate to Relationships or Adoption or Infertility or Special Needs next. And what will be the response to that? 'Tough shit: you shouldn't have been talking about personal issues on the internet'?
Whole sections of MN presuppose people sharing difficult, sensitive, often hurtful personal experiences. To talk about posts being or not being 'in the spirit of Mumsnet', and moderate or criticise comments for being unsupportive, and give the impression of community (and I'm talking about MN users as well as MN admin) -- and then respond to DM story-grabs with a snide 'well, where's your membership fee?' or 'did you think you were amongst friends?' or 'so just don't post any more, then' is a total attitude-switch.
I know there isn't one single easy answer for the issue, but it's depressing to see that the kneejerk reaction to this issue is to pour scorn on anyone complaining about it. Yes, it's naive to assume privacy. But these aren't isolated incidents; the DM really seems to be here more and more, and how we react now risks setting a precedent. I would hate to see the more sensitive topic boards left completely silent and unusable.