YouTube is perceived as a family-friendly, PG to PG-13 brand. It would be a very different, and much more niche, environment if it were a free-for-all of pornography, obscene language and graphic violence.
I agree, but notice that except for pornography, which I forgot to include, I covered all of the others (sort of) with "hate speech". Banning violence/insults/pornography/harassment is an obviously useful filter for a PG-13 site, but I don't think it qualifies as curation. To me curation implies something related to the selection of quality.
