They can police this, and take out the loopholes. Look at how social media executives have dealt with the right-wing, and left-wing when it comes to Youtube, Reddit, and Twitter to a lesser extent.
On Youtube, they have marginalized "borderland content" which is basically anything CNN, MSNBC, Fox News, and big corporations don't like. You get fewer views, and subscriptions, and lefty videos always automatically transition to mainstream media. If I watch TYT I get MSNBC on the next video on autoplay.
On Reddit, they have banned all of the "violent, and brigading subreddits", and are on a censorship spree. If you're a neoliberal or neoconservative your ideas can wreck the global economy, but the Winklevoss twins don't like it if you said the word guillotine.
On Twitter, you can't say curse words at public figures in your tweets. So now you have to start using conspiracy theories against billionaires, and public figures you don't like, and the mainstream media is about to do away with that as well by pressuring social media.
If the dark web had lefty forums and was more accessible & popular I would already be on it.
Oh, and the reason they are doing this is to attract investment, and to have advertisers come to their platforms.