As someone who has worked in a theme park, (granted it was Six Flags, but still...) I know what line cutting is and isn't. It's pretty easy to understand.
If someone has to leave the line to go to the restroom or some other necessary reason that allows you to function as a human being...(ie...needing water or something so they don't pass out from heat, ect.) that is not line cutting. You are not impacting the wait of the guests behind you.
If you are coming up to a line that you haven't previously been standing in and try to join others that have been standing in line, no matter what the situation, teenagers, parents with children, etc that is indeed line cutting because you haven't been in the line previously and you are therefore impacting the guests wait times behind you.
As for Fastpass, you are not cutting in line because as Disney says, when you get your Fastpass, it is saving your place in line and making you wait to ride the attraction at a later time. Your wait time has been prolonged, therefore, you are not cutting in line because you've had to essentially "wait in line" by doing other things. (Be it other attractions, eating, or shopping.) Also, it is not considred cutting in line if the theme park sanctions and creates it.
As for the Guest Assistance Card...that is more of a sticky issue with many, however, to me, it is not cutting in line because they have enough they have to go through in life. And that's all I'm going to say on that matter.
In the example of the tour group having 20 people that don't know about the guest assistance card, sure, it could be feasable, however, more than likely, not all 20 people have some sort of disability that would require them to not be with their party originally. Even if they had someone with a disability, 20 people are not necessary to hold back to wait with the disabled person. Matter of fact, I believe when people do use the Guest Assistance Card, there is a limit to the number of people that can ride with the disabled person.
Finally, you also have to look at the number of people joining a group. 1 or 2...I'm more likely to be a little leinent. 4 or more....when you get those kinds of numbers, those are people that should have known better and had their party wait for them before they got in line for the attraction.
It all comes down to common sense.