Agricultural runoff is a BIG deal here in my neck of America. Soap? not so much.
lol
I don’t think they are overly worried about the soap but about what else might be carried in the water flow as it makes its way through the storm drain system. But even soap in large enough amounts (which can be small amounts accumulating from different sources) can be a problem which is why quite a few State and National Parks have rules about washing in the lake and river water found there.