If it's not your property, dumping anything is illegal. The lack of harm it causes isn't exactly relevant. It's much like digging up plants. You could claim taking one itty bitty mountain laurel ain't hurting anything. But then you'd have to have a complex list of things you can take, or can dump, and in what amounts. Imagine that nightmare if you are anti-regulation. Sometimes the rules do make perfect sense. Other times.......not so much.