I think the problem here is pretty obvious: we have laws and norms about hate speech, but why do we have nothing to prevent hate reading?. Why do we allow people to seek out and absorb hateful ideas and opinions? It's the intellectual equivalent of financing terrorism and the people engaging it are escaping justice. It's a huge blindspot for society. Who knows how much harm is caused by this 'dark thought' in every day life, as the rest of us unwittingly engage with these individuals, assuming they are not motivated by any sort of forbidden information.