I agree with dirty. A little consideration goes a long way, on both ends. There are asshats, to be sure, but a lot of times videotaping with a phone is a great deterrent. If I were fishing that jetty from shore, and that guy came and parked on my line, I'd get out the phone/video camera.
On the other hand, people DO take advantage of the "watch out for shore fisherman" card. Just like a pedestrian, just because they have the right-of-way, doesn't mean that they don't occasionally get run over.
At the end of the day, nobody owns the river. Or rather, we all do. (cue the Kumbaya music)