There are vast reasons to prevent brigading, false information, information manipulation and any other number of malicious inputs that otherwise in a forum for people require trust to maintain community quality. Even for free reasons, especially say nice interest groups like one I used to be on for small satellites that got destroyed by uncontrolled bot spam.

I do not agree that every piece of the internet "should be" behind a paywall because bad actors exist. That world is the literal death of the "open" internet, putting everything behind a paywall.