The users ARE Publishers... If the users are US citizens they have rights.
“Risks Associated With Publication.
Every time you >publish something online<, whether it’s a news article, blog post, podcast, video, or >even a user comment<, you open yourself up to potential legal liability.”
Media Law:
https://www.dmlp.org/legal-guide/risks-associated-publication
“Like other publishers, FR prints a disclaimer about opinions and does not claim copyright over what’s published. But it’s still legally the publisher.”
Wrong. That is why there is a disclaimer here... They DO NOT want to be considered as the publisher of the content which makes them liable for it. The users are the true publishers of the content. All they do is provide the public platform so the member users can publish here.
This is a well known industry standard and legal point. As a domain owner myself I am well aware of this reality.
The fact that newspapers/online sites put out a disclaimer that someone's opinion is not theirs or that they hold no copyright on the material is irrelevant to their status as a publisher, only that they should not be held liable for the content of what they are publishing because it's not theirs.
Perhaps this is clearer: FR has many specific restrictions on what can be reproduced here from different news sources. FR clearly does not own that material and FR is not responsible for the post itself.
It is, however, liable for a copyright infringement as a publisher if those rules are not followed and it does not remove the article.