There are lots of other bodily fluids that end up on the pitch at all times.
Blood, Vomit, snot, saliva and all these without issue as generally the player is doing so unwillingly. The Pee is no better or worse than some of these, but I feel the fact that they have chosen to do it on the pitch, instead of walking a maximum 35m away and doing it off the side is certainly ungentlemanly conduct and would deserve a red card just for the disrepect factor, let alone the illigal factor. And I don't believe any Judiciary would find a Red card for pissing on the pitch harsh.
I would probably have the same issue with a sub who was on the sideline walking on to the pitch to throw up. He is perfectly capable of walking the same distance in the other direction.