The problem is this;
If one person is not a big deal, then surely two people wouldn't be a big deal either. And if 2, then how about 3? And so on and so on. At some point it becomes obvious why this isn't a good idea.
It's like littering. One candy bar wrapper or cigarette butt isn't a problem. The problem is the # of times that this is done. Now, the only reason people don't take showers in Walmart parking lots is probably because they don't realize they would get away with it. But this guy is his own worst enemy; showing others how to do a thing will over stay his welcome to do so on the sly at Walmart.
Besides, why at Walmart? Why not down the road and on some dirt area where the water is absorbed not a puddle in a common area many people are going to be exposed to? I'm sure there are plenty of grassy areas he could have parked and the water / waste issue would not be a concern.
So, unless you are willing to have dozens taking showers in walmart's parking lot, I'd say it is not a good idea to advocate this guy's practice.