Almost each river has its own proper ettiquite.
Around here on the smaller streamers as long as your are about 100ft apart or can't cross fly lines you are fine.I do not think car claims a spot, but it is in good taste to ask where they plan on fishing and yield to them.

I view your example as your friend and his son were fine in what they did. The guys who were working down stream should have gone around them when they got close and not disturb them. Alot of times on a small hole asking someone if you can join them can get you in trouble free.

I don't take crap from anglers who have little ettiquite, I always call them out on it. Alot of times that person is used to fishing a stream back home that anglers are packed in very tight and see nothing wrong with what they are doing. Most back off and apologise, and on more then one occasion enjoyed a nice day of fishing with that person.

I have only had 1 nice conflict call someone out on bad ettiquite and it was a goodie. I was fishing a kinda busy spot and fishing to a nice fish on the far bank that was just within my range but had hard current to read. I noticed someone walking upstream behind me and watching the fish very close. I pull my fly in to change it and he went 15 yards up stream from me and threw a cast to the fish. I slowly changed my fly and laid a cast about foot from his knees and asked him if he thought we were too close to each other. I think it was the last thing he thought I would do and he quickly backed down and left the river.

Joe Fox