I rarely dump my gray water in the forest and I have personally only ever seen one other person do it.
You act like everyone boondocking is dumping their gray water in the same place all the time. People aren't draining millions of gallons.
I've been boondocking for years because I loath campgrounds and I've never ever been able to detect that anyone has ever drained their gray water in any of the campsites that I have ever stayed in. If they did the campsites would most likely be even greener than they are.
I would have to think that if it was damaging the environment that the Forest Service would have put a stop to it years ago or we'd have a bunch of tree huggers blocking freeways and tearing down Teddy Roosevelt statues all across the country.