It probably has something to do with the old adage, "Give them an inch, and they'll take a mile". If dumping grey water on the ground was endorsed, then many campsites would become mud holes. Having to dump grey water at a dump station provides an incentive for people to use the campground's shower facilities.
As far as tent campers go, we do not have showers so there is no shower water to dump. My dishwater goes in the fire pit. If I had to transport every pail of water to a dump station, I would quit tent camping.
Plus, rustic campgrounds often favored by tent campers have no dump station to begin with. So now that I have pumped a pail of water to wash my dishes, what in the heck am I supposed to do with it when I am done? Surely cannot dump it in the outhouse.