Are you saying that they aren't exempted from responsibility because they are not responsible, but because it's better that they not be held responsible?
Why would they apply to publishers more generally if it was about lack of responsibility? If you recall the decisions about the Communications Decency Act, it states that - "No provider or user of an interactive computer service shall be treated as the publisher or speaker of any information provided by another information content provider."
This means that people that own or operate forums are not treated as publishers of the content on forums, and removes the responsibilities of being such a publisher. The act was passed in part because providers said it was injurious to force them to regulate all content - they shouldn't be responsible because they couldn't regulate content without massive cost.
Now you're saying that they should regulate all content - ok, but if that's the case then they should be treated as publishers of said content, since they can decide on a case-by-case basis what is and is not published on their platform.