Leaving pet waste behind may be the result of an unprepared human escort, or an act of ignorance, defiance, or apathy. Determining the prevalence of leaving behind feces might be the first observation point, and asking those who leave behind the waste whether it was the result of being unprepared, ignorant of health code, defiance or apathy toward social acceptance, or just not convinced leaving the pollutant behind is truly resulting in a measurable impact on the environment.
I have noticed that many parks are now making it easy for people to clean up after their pets by having stations with bags and a trash receptacle. I think making behavior changes easy and obvious for people does help in many cases.