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.
Unless you're convinced that there is never a legitimate fair use case, feel free to substitute whichever you like.
In general, the reason copyright enforcement is pushed for is not accurate enforcement of laws, but rather maintenance of business models while paving over "minor details" such as legitimate contracts and licenses between copyright holders and others.
The digital copyright regime has effectively paved over centuries of intricate law to create a binary of "free" and "nonfree", with no internal distinctions or intermediates. That this is not widely recognized is a sign of how effectively media conglomerates control perception of the issue.
You simply can't justify unilateral global takedowns on copyright grounds. Many legitimate countries disagree about copyright rules, and they just aren't so simple as a global "copyrighted?" flag. If you want to be a global moral police, you can justify blocking hashes for those reasons as long as there are no collisions and everyone agrees, but that doesn't sound terribly likely either.