The obvious question is then "how are they helping pay for the servers they're using?".
It's not that I don't see your point, everyone should be able to take part in a community without having to spend money, but I do find it annoying that whenever the topic of money comes up, we end up debating the hypothetical of someone with 0c spare in their budget.
Charging for membership worked well for Something Awful, and they only charge something like $20 for lifetime membership anyway, plus an additional fee for extra functionality. But you don't get the money back if you get banned. Corporations would still be able to spend their way into the conversation, but it would be harder to create massive networks that just flood the real users.