I think the problem is this whole 'is underage nudity art or pornography' debate.
Yes, its a drawing, its artwork - but some people could argue having a naked seven year old girl posing lewdly as art. What is the difference between a drawing and a photo? Obviously, the photo needed a real live little girl to debase herself for someone elses gratification - the drawing is just drawn from mind, nobody was harmed in the making.
Some people would make the arguement that the second option is the better, because nobody was harmed in the making. Sure, fine, i'll agree with that - but unfortunately, there are a lot of people out there who would take the next step. The same people who would play or watch something violent then do something violent in responce - and yes, there are people out there that do it, dont start whinging just yet.
I'm not saying everybody would do it. I'm not saying that if i suddenly whipped out a picture (real or drawn) of a little girl naked you'd automatically become a pedophile, just the same as if i handed you a picture of a gun would make you want to shoot a whole bunch of people. But, unfortunately, there is a small percentage that would take this photo and...take the next step.
These people are sick. These people need help. And, unfortunately, a lot of people in power look at the whole thing in the mindset of 'omg its gross and disturbing, lets just put a blanket ban on everything to make sure this doesnt happen ever'. But then, zero-tolerance policies never work for long unless rigidly policed, so...
Anyways, i think i'm trying to say that people in power think they are doing something for the greater good, and they think that by keeping something off us all they will stop the select few amongst us from doing something we may - or may not - regret later down the track.