I'm a bit confused - Cars get dirty as they're outside, which is the environment. People wash the cars to remove the dirt. No more dirt is entering the environment, it's just being moved off the paint work.
Ok - so oils / chemicals from soaps etc... are not natural but how diluted must these be from average joe washing their car once every few months. We've got to be talking non-measurable quantities of these chemicals entering the environment.
I'd be more concerned about over cleaners, bleaches entering the water from inside the house...
This is quite simply not a problem IMO. A detailing company, using concentrated chemicals daily is a different matter.