The fuss isn't really necessary, though, as it comes down to simple definitions set by US law:
* A person (or child/individual) is a member of the species homo sapiens
* A minor is a person under 18 years of age
* Obscene visual representations of the sexual abuse of children involve the depiction of minors, or those who appear to be minors
If it doesn't appear to be1 homo sapiens, it's not legally a person,2 and so not a minor.
That said, it's still possible for the work to be obscene (a word with particular meaning), but only in the sense that any art can be obscene by passing the Miller test. Unlike the law above, simple possession of obscene material is legal, but you can still get in trouble for distributing, importing or transporting it - including over the net.
Now, whether or not such material should be specifically specifically outlawed, regardless of obscenity . . . personally I side with the courts - who view it as freedom of expression without a victim - but I can understand why it squicks a lot of people, and why (in this case) a convention might have issues with it. However, they seem to have only had problems with the legal risk, so it'll be interesting to see how it works out.
1Where the intended meaning of "appear to be" is "virtually indistinguishable from"
2Yes, a company can also be a person . . . but it can't have sex.