It does really boil down to the difference in impact from one person or an occasional person vs the large number who will visit the park and camp in the same spot. The cumulative problem is where the issue comes in.
One can argue that the impact is insignificant or not very harmful in the individual case but in the cumulative the argument will fail. Also if one can "dump" a
solar shower then why not drain the gray water tank on an RV after all it is the same sort of thing. The answer again comes down to cumulative harm of 35 gallon tank compared to the couple of gallons used in a shower.
Not really any good way to differentiate between family of 4 using some sort of portable shower in a cabana tent with the water running across or into the ground or a family of 4 draining the holding tank after showers.
I seldom if every use more than a small aluminum coffee pot of hot water for dishes, maybe cut with a cup of cold to wash and the straight hot water as a sanitizing rinse. I can tell you if you toss that into the bushes near camp in the Huron National forest you will find "visitors" showing up, if nothing else it draws flies. If you are less lucky racoons or skunks. Now in the forest one can dig a "cat hole" to pour that small amount of water from dishes but if one was in a rustic campground used by a hundred families over the season that would probably end up a mess.
Even small bits of food will draw rodents, and where the rodents congregate so to do snakes. Snakes generally do very poorly around humans. And in parts of the country where venomous snakes are more common dogs can do poorly around snakes.
It is true that most rules are written to prevent people doing something that left to their own devices people would want to do. One wouldn't need penalties for violating fire restrictions in a drought if there weren't folks who would have campfires despite the fire restrictions.
I'm thinking on a longer trip I may need to think about how to have a water collection tray or basin that I can dump in a portable gray water tank for showers. At least something I can hang my head over to wash my hair. Because I know I will be camping in places that restrict dumping of wash water.
Some parks apparently also frown on people putting up a cabana tent for a place to pee on the ground. Guess that is a real problem that rangers deal with. Considering what happens to the bushes in the park where the dogs all go.... I figure aside from the ick factor, and potential odor, if enough people do it in the same location it could kill a good size tree or vegetation in a season. Especially in the more arid areas that lack enough rain to dilute the ammonia. In our yard the grass does well around the spot, but the spot tends to die if we have a dry spell.