I agree with your point where it concerns interactions in most social forums, but strongly disagree with your implied suggestion of a social norm for Less Wrong. I prefer a community that produces the given examples to one that produces your suggested corrections.

The karma system somewhat mitigates the "silent approval, vocal dissent" phenomenon here, though I agree with NihilCredo that there still exist confusing side effects, and that a more nuanced system would be superior. But as it stands, the community norm for an author of a post should be that criticism is meant at face value without further implied disapproval, and that the post karma is the better clue to readers' overall impressions.