I wish I had the energy to delve into this but I am extremely drowsy right now. But I have seen what you are suggesting played out firsthand on numerous occassions. One I can mention here, not pornographic but still answers your point, but there was a time when "child model" sites we very popular. During this time- I can even pinpoint this to late 2002- John Walsh had a daily talk show. One episode he actually had the model known as "Lil Amber" as a guest. To her credit, both she and her parents stood their ground. But John Walsh claimed that showing examples of her work would be "too graphic." Anyone familiar with Lil Amber would gasp at this assertion.
I remember reading an interview with a comic book artist years before this. Comic books have a code against graphic images. He did an issue of Green Lantern which involved a woman being killed. He initially depicted this. But it was not code-approved so he had to redo the pages with the murder happening off-panel. He claimed that by all accounts of everyone who saw both versions, the original- in which we actually see the murder- is far less chilling. He explained this is an old cinematic trick often borrowed by comic artists. The imagination is usually more graphic than reality.
So yes, it was an intentional move on John Walsh's part to not show any pictures and lead the audience to believe Amber was involved is some seedy Satanic ritual or whatnot. Knowing enough about Walsh, if he actually detected anything "inappropriate" about Amber's site other than his own personal distaste he would have called CPS rather than put on a show. Well...probably both...
But yes. The strongest defense for CP being illicit is mystique. I also believe part of the reason is to prevent folks from realising exactly how common paedophilia is.