As you say, it's always been possible for someone to deliberately betray someone's confidence by copying and pasting. The real problem with the new feature is that it makes it incredibly easy for those who would never dream of deliberately posting something confidential to do so without thinking or even totally by accident.

I do agree though that harshly worded requests are not the way to go. If the person posting doesn't trust their LJ friends, then they should de-friend or set up filters so that only certain people can see their posts.