Your analogies don't work. With drugs, producing and using them are crimes. The intent and act of viewing Child Porn is the same - producing or using the produce is the crime.
If someone downloads Child Porn, or knowingly copies it from somewhere, this could be giving someone the idea that they better keep producing it.
Maybe I haven't made myself clear. I am not arguing that "using" child porn is not a crime; it plainly is, in my country at least. I don't think it should be, though, as I have said. Its production should continue to be a very serious crime, for obvious reasons. For what it's worth, I see nothing immoral about drug production or use, and consider drug prohibition another pointless and destructive law.
As for your second point, yes, it might. Or it might not. Either way, speculation such as this does not belong in a legal setting. And if you take your thoughts to their logical generalised conclusion, watching anyone do anything bad at all makes you partially responsible. I don't agree with that, of course, and I doubt you do either.
Anyway, I've said all I'm going to say on this - I was just trying to make a point, I'll bow out before things turn any nastier.
I'm not talking about viewing illegal things as a crime, and I don't agree that my arguments logical conclusion is that watching anything wrong makes you responsible. I'm talking about production and use. Watching a video of taking drugs is not using drugs, it's therefore not a crime and won't encourage drug producers to produce more drugs. Watching a video of Child Porn is using the produce of an illegal activity, could very well encourage more production, and is therefore a crime.
ErrantX's job is to determine the intent of the viewer by seeing if they actively downloaded and used the produce of an illegal activity. If he does his job well, I see no problem with putting people in jail for this crime.
If someone downloads Child Porn, or knowingly copies it from somewhere, this could be giving someone the idea that they better keep producing it.