Many people throw rubbish in a certain place because that is the norm. Once, we see a pile of rubbish, we assume that that is the designated place, without questioning whether it really is.
This issue is common in other places too, where they take a normal place as a dumping ground. The image below is of a small road with the sea on the opposite side of the image. This board states that this land belongs to PELINDO, a harbor company in Indonesia, warning that it is prohibited to use this land without the permission of the company. However, it has obviously become a common dumping ground.