So, if that's the case, why couldn't you just allow the shower drain to a hose and allow it to drain away from the site?
Bikerman,
Your thoughts/theory are reasonable.
Gray water free of harmful chemicals and soaps...is not harmful.
If you camp out in the wild, no big problem with a few gallons of water.
If you camp where others camp (most of America) logistics pose a problem.
If each camper dumped just a couple gallons over a few days, then left and the next camper didn't arrive for a few more days, it wouldn't be a big deal.
But...though most of the teardrop crowd would likely drain only a few gallons over a weekend visit, some people, like those with 5 kids and a 37' RV with 6 faucets might drain 30+ gallons per day X 2-3 days.
They pull out, and the next camper pulls into a mudhole.
Possibly a smelly mudhole from all the food scraps they put down the drain.
Multiply this potential x the number of spaces in the campground, and the number of times a year a new camper pulls in, and you have a reason to ban dumping gray water...
Rob
PS. I know you said "drain away from the site" but it only takes a few that have no concern for the next guy that ruin the opportunity for the rest to self regulate.
People would do that? 
I got it! 
