I don't work at Disney, but I do work for a nation wide department store. The reason in that store that people do not interfere in many cases is not because "the customer is always right", but because they value your safety above everything else. (Which I'm sure their concern isn't about my safety but about not being sued)
For example: If a kid is reported missing by a parent, the station people all all doors and begin a search for the child. HOWEVER, if you are at a door and see the kid being pulled out of the store, you are not allowed to follow after. Their reasoning is, you have no idea what the guy has, and maybe he'll pull a gun and kill you. I know that I personally would say "screw the rules" and go after the kid, because I would never be able to live with myself afterwards if something happened to the child. But I do see their reasoning, even if I don't like it.
For other situations not as drastic, they don't want you to interfere either, but want you to get a manager or security. The reason again is your own safety. Sure, someone may just be yelling in line or doing something rude, but how do you know how they'll react? I had a man one time stand there for over 5 minutes telling me to "shove it up your *****" just because he didn't like the price of something! They would rather get the person out of the door, then put you at risk. There are so many odd people out there who may go ballistic on you and hurt you physically just for the mildest events, that they don't want you to take that risk.
Again, I don't know how Disney is, but perhaps the reasoning is the same; there are many cases where their employees would be at risk by confronting these rude people, that they don't want them to take the risk.