So for example, I'm sitting next to a teacher waiting for a train and reading the newspaper. The teacher sees a story and says hey, can I have that when you're done so I can make some copies for classroom use?

That kind of copying is clearly fair use, right? But the same thing happens on the internet with some kind of hash-based copying prohibition and the teacher can't copy the story from me. Because the computer has no way to know that the law allows the copy. So it can only allow everything or prohibit everything.

That sort of system can't work. It doesn't have the information or context or logic necessary to make a fair use determination. But "prohibit everything" is exactly what Disney et al want, so they're always pushing for it anyway.