Paintings, sketches, cartoons, etc. of children and of a pornographic nature are legal at the federal level so long as they have artistic, cultural, or [something I've forgotten] merit. Or so I believe - not a lawyer.

Anyway, to make the point in a more straightforward way:

What ought and oughtn't be allowed expression according to the principles of freedom of speech is not a clear-cut thing. There are many different views on it, and to dismiss all those with views different from those who wrote US law is silly.