I guess it may be the lack of enforcement because its rare to get licenses checked on river. Probably because game wardens don't want to track on foot.
The same here... I had renewed my licence all these years, and in 10 years, I NEVER saw anyone checking them out... On the other side, as the sea is relatively close, not many people go fishing on lakes... People go there for a walk in nature, and in that way throw garbage around... But, the water (in lakes that I visited) is relatively clear...