That's not how intellectual property works, at least not if the TOS of the website have specific rules on how ownership is handled. Afaik you retain copyright to what you put on reddit. And yes, even short comments fall under that.

But also, posts that are long analysis, personal stories or fictional content are posted on here- just because it's a public forum doesn't mean it's a free for all and the content on this site can be used commercially. You're free to use the site and have access to all this content other users generated. You're not free* to compile your favorite user's post history into a book and sell it.

(I've also seen this argument of "It's in the public forum, it's available to the public" used by art reposters, who steal artists' work and reupload them here and on other sites. It's a really shitty way of compartmentalizing intellectual property imho.

It might not be apparent on a site like reddit, because so many users on here don't use the site for "serious" work- though obviously there's also lots who do. Even when the original creator doesn't make money off of their creation, be it writing or art, there's a certain lack of disrespect in that kinda thinking for the content creators who can't even get the courtesy of credit or direct support.)