Obsessed,
I think you're right.  It IS aggravating when someone encroaches on your fishing "zone", but I'm not sure you could say it costs a guy many fish.  It's like being cut in on on the freeway.  You're pissed about it (at least I am), but in reality you're only one car length and .32 seconds behind where you just were. Is it worth getting all bunged up about? Of course not, it's just the lack of common courtesy that gets you off-kilter.
There's going to be pricks wherever you go, the freeway, river, or grocery store (yeah, I know it's 10 items or less, but I've only got 16 items....blah,blah).  I know a lot of strokes could be avoided if you didn't let those vermin under your skin, but it's a challenging world out there.  Hell, I'll let some guy into traffic as a courtesy, and then find myself getting aggravated at him for doing the SAME THING I just did if he slows to let someone in.  Go figure.
All I can say is a lot of anger could be avoided if people would just talk to each other.  "Hey, man, I'm planning on FISHING that tailout....."  will usually get a guy out from below you.  "How far you want us below you before we start fishing again?" The dude you pass will likely just say " Oh, a ways down there should be fine......" and nobody gets ticked off.  If he says, " Oh, the takeout should be far enough...", then maybe he's a guy you wouldn't feel that bad about pissing off a little.
"Common sense" only IS common sense when you know the river protocol.  You know: pass to the inside of drifters unless they ask you to pass in front or don't give you enough water to pass.  Don't start fishing immediately below someone you just passed.  Don't pull into a 100 yard long  hole during fall Chinook season and think you're going to have it to yourself.....Pick a section and work it.  Don't move up or down to stand closer to someone who just landed a fish.  Don't look down into a hole someone is fishing from the opposite bank, you think they want the fish they're after to see you?  And last, never go up to a guy who's gently handling a fish he intends to release and say, "Hey, if you're not going to keep that fish, you mind if'n I keep it?"  Do that, and you'll be called a cracker for life!
Sorry to ramble,
Fish on................
_________________________
She was standin' alone over by the juke box, like she'd something to sell. 
I said "baby, what's the goin' price?" She told me to go to hell.
Bon Scott - Shot Down in Flames