In 1996, Congress passed the Child Pornography Prevention Act, which made it illegal to own or distribute anything that appears to be a pornographic image of a child under 18. This would include an otherwise-innocent photo of a child, manipulated to appear to be nude and/or engaged in sexual activity, or a sexually-related photo of an adult intended to look like a sexually-related photo of a child. Virtual child pornography is, by definition, an illusion.
The most compelling rationale for the law was that authorities can't easily tell the difference between real child pornography and virtual child pornography. The counter-argument is that it's the prosecution's burden to prove an illegal act has taken place.