Keep in mind that when one speaks of "illegal" or "legal", that isn't an accurate representation with regard to pissing.

The issue is obscenity. In some states, any defecation or urination has been ruled obscene, and in many states, you would probably have a very hard time defending against obscenity using the three prongs of the miller test (average person offended, appeals primarily to prurient interest, work as a whole lacks serious artistic/literary value.) So there are more do-not-ship states for pissing than for regular porn.

And keep in mind that the justice department is going after stuff that would be fairly easy to have the average person offended about ... rape, violence, etc. Scat and pissing aren't, in my view, that far away in terms of the average person being disgusted or offended, so that's why we are uncomfortable with it.