Don't get me wrong. Trust me... I am MORE than capable of standing up for myself and not being steamrolled. My thing is that I have been learning to pick my battles, as I have died on many worthless hills in the past, and trust me, I fight to the death. (figuratively speaking, I am actually a very nice guy)
However I do understand your premise that being polite helps diffuse an already awkward situation, whether it be a line cutter, a rogue kayaker, or a squatter.
But the notion is that if those people did not give any consideration that there was a waiting line, or a fisherman fishing, then they probably will not give much thought to a polite statement either.
I have found it easier to just move on, because in the end, most of these people are only thinking of themselves and it is not personally directed towards me, and I only have so much time to fish.
With that said, get between me and my Nascar...its game on! LOL!