Well, largely for the reasons I've already given. Plainly put, its unhygienic, unpleasant, unhelpful, potentially damaging, disrespectful to the artist and anyone who holds such art in high regard and, perhaps most importantly, has no measurable benefit. We're talking about previously unrecorded artwork here. If it was going to disappear overnight, ok, take whatever measures necessary to view it and record it. But it won't vanish that quickly, and if you want to see it that badly, make the effort. Besides, after discovering it, what's the next step? Reporting it. Imagine the conversation with the FLA or EH:
FLA: Wow, that's great. Thanks for letting us know. Does anyone else know about it? Did you get an image?
Me: No, bad light I'm afraid.
FLA: Ah well, we can go back later and record it more thoroughly. Did you do anything else?
Me: Yes, I pissed on it.
FLA: ....

Pissing on an ancient piece of art for the benefit of being able to see it better is, quite frankly, selfish. If someone cares about heritage like rock art enough, they'll report their findings or go back to view them in better circumstance. Either way they'll get better analysis. I don't know what the effects of urine will have on the stone, but one thing is for certain - it won't benefit it. That's why.