It's like being a respectful guest. If I attend a soirée at someone's house and leave a pigsty, I won't be invited back. If I invite friends to meet me in the park, I'll make sure the area is left clean when we leave. If someone doesn't pick up their own napkin, I'll do it - one it's respect for having use of the property and two, I don't want to loose a privlege. If I become known for causing problems, I'd anticipate being barred from the premises. Surely among the homeless, there are enough of them that they could clean up after the lot.