But if there is no way to remove content because of mandatory free speech, then the controversial content goes to 99%. No advertiser will pay for ads alongside a torrent of profanity and porn. It just won't happen. (Well, porn sites might? But no one else.)
Not to mention the fact that the sites could not stop advertisers from posting ads on their site in any case. (Since it would be illegal to remove content. Free speech and all that.) So why would I pay that 8 figure yearly sum to you that the big advertisers are paying today, when I can pay not even a million to a spam farm to post my ads as standard comments that you are forbidden from removing? And it's completely legal.
I just think you're being a tad idealist. Spam farms exist. Botnets exist. Pedophiles, porn stars, klansmen, all these exist. This stuff would be the majority of content, not 1% of content. Spam alone would overwhelm interesting content, and that's before you even throw in the porn, pedophilia, and klan rallies.