I am thinking from a piracy perspective. If I share a link that contains a book, what can be done from DCMA or legal regulators? They can't ask the server (textarea.my) to remove the link because it doesn't exist.
They can't track every website with the link and ask to be removed, either.
Could they ask textarea.my to not parse the link and thus, not display the content? Could textarea.my refuse?
I would hope not. The copyrighted content seems to be the link rather than anything in the app.
Your example sounds like stopping notepad from rendering copyrighted content
From a technical perspective, you're absolutely correct.
From a regulatory perspective, it seems unlikely that most courts would appreciate the difference. In their mind - you run a website, and that website contains copyrighted content. Take it down.
You'd probably have to just blacklist the link in question to avoid a legal headache.
In this case I'd say the link is the content. So it would be the place where you share the link, rather than the "rendering page", which should be more worried
A book won't fit in the URL anyway even with compression
Someone in another comment posted Crime and Punishment by Fyodor Dostoevsky, so a book fits in the URL. It is just that the URL is ~500000 characters :-P (the book itself is ~1.2M characters)