In my experience, you're right... most situations can be handled politely & easily. I find that most people are simply confused first-timers who aren't trying to be rude, but don't understand that others have sacrificed large amounts of time sitting on a curb to guarantee their spot.
I'd like to believe that, but I can't. Anytime we have had this happen, and someone (either myself or someone around us has said anything) the rudeness has only ever been treated with escalating rudeness. Like tour groups and line jumpers, these folks know what they're doing. They are using others' goodwill against them, taking advantage of the fact that people won't raise a stink or take them on.
Don't get me wrong, I'm not advocating for or against confrontation (although I personally chose FOR, simply because I am very impatient with people who feel that bad behaviour will get them what they want), just saying that you will never win with people like that.
But for every one rude person we encounter in Disney, we find three more willing to hold doors, share tables during a crush, watch out for 'lost' little ones, and start a friendly chat or share Disney 'secrets' in an overly long character line.
I just wish there was a way to weed all those self-righteous guests out. They really can put a damper on your day, whether you want to get into it with them or not.