You really struck a chord with your experience. This year I was able to fish the Roaring Fork River in Colorado on numerous occasions. I experienced three pretty outrageous incidents of poor etiquette.
The first involved a guide that has been around for quite some time and has a reputation for walking in on folks. He walked in on me, less than 25 feet away. When I said to him that I was obviously working upstream to that area he responded by saying "what do you want me to do - go upstream a mile". After I responded "preferably" he moved further upstream by 10 feet! What is particularly disturbing is that his client appeared to be a novice and in future outings this new angler might think it reasonable to do the same thing.
During a BWO feeding frenzy an angler burst through the willows alongside the bank that I was fishing to many rising fish. He looked upstream, then downstream, and then promptly walked through the fish that I was casting to. When I pointed out that he was not being courteous and had put down the fish, he responded with "well its public waters". I was so flabbergasted that all I could do was shake my head.
The last experience was in mid October when the crowds are non existent. Fishing with a friend who was upstream of me by about a hundred feet, a film crew just plopped down on the stream in between my friend and I. It was during another hatch of BWOs and PMDs and the fishing was fantastic. While fishing I watched the "star" trying to catch some of the risers, all the while exhibiting film worthy form. After 20 or so minutes, I finally moved upstream to catch up with my friend. As I passed one of the cameramen, I inquired who the pinhead was. Said he was some famous shark fisherman from Southern California. I told the cameraman that I understood the need for a good shot, and good light. I told him that if they had merely asked if it were alright to go in there that I would have not only yielded the area, but that I would have shared some of my flies that were working. I am not sure if he understood!!! As it turned out, I saw him finally catch a "dink" Relating this experience to one of the local guides that I often fish with on his days off, he told me that crew had popped in on he and some other guides on a pool downstream earlier. While they all were catching, the star was not. So perhaps there is some justice and payback for thoughtless behavior.