I think freedomfrog has the right idea. It is a way of rebellion. The artist know that those above them do not edit this stuff. Some people in the know have pointed out that alot of stuff gets through to the people without editing. That is why two pieces of literature may contradict one another.
I have always made cartoon quips out the pictures. I show them to wife and she can't keep from laughing and has to go out side. All my study books and magazines were full of my doodles, art, and jokes. I have some good stuff even if I say so myself. It is one of the ways I kept myself sane for two hours. And my little way of showing my opinion of the material.
I doubt there is any sinister purpose behind any of it. The literature is looney enough.
The artists can even deny they are doing this on purpose because the society says it is just coincidence. The artist are probably having a good laugh.
There are some very good artists who do this kind of deliberate hidden art. I have a collection of very good ones from the internet. It takes a unique talent to be able to do this.