Technically, I suppose it is but you'd have to see the lay of the law where your server is hosted (which is what would probably apply). It may not be in the letter of the law but you'd have to check into precedent as well. Even if the law doesn't specify that online comments are covered by copywrite, a judge could have ruled that they are.
Personally I have stories posted online that I would fuss about but I can't really see me fussing about forum posts (I don't post on non-forum social media much so forum posts are most of my online output).