But children are incapable of being consenting performers. That's what separates abuse material from porn. So to make damn sure there isn't any overlap in the Venn diagram of media that depicts sexual acts, they'd rather not associate "child porn" with anything else that exists in the universe of "porn".

It's a completely separate category, not some "bad" end of a spectrum.

That's the reasoning I've heard before, anyway.

And I agree with you on the reverse treadmill thing. It's interesting. On a related tangent: I've always hated how journalists use the term "sexual assault" to refer to a wide range of offenses, from forcible rape to a passing grope. Although those are both bad things, it's clear that one is tremendously more harmful than the other. We should use language to clarify that.