Honest question, other than the fact that courts haven't actually said so, where is the difference between an extremist video including acts of violence and the material that is already illegal?

Created by harming others? Same. Is a threat to society? Same. Goes against cultural norms? Almost same (those in the US are far more accepting of fake violence, but real videos of people being murdered tends to be quite taboo).

One includes violence, the other includes sexual violence. But any argument for banning or allowing one seems to be a very close fit to the other.

I don't intend to put up a straw man, I'd like to know why you think my examples are a straw man.

Content classification is something you must do if you're a major content provider in the US, period. Saying someone should just "let the advertisers sort it out" means you're not addressing the type of content classification we're discussing here: per-user, not per-advertiser. Porn & CP are extreme examples, but so are violent extremist videos.