I get your point—legal documents—briefs and opinions and the like are like clip art that you can grab, cut-and-paste whenever it suits. Many contracts I imagine, are chains of verbiage culled from previous documents. It makes sense that the profession would allow this: there’s no ambiguity and less chance of the intended meaning slipping outside of its lane, so to speak. I can imagine that a single word or punctuation mark might completely change a legal document.
However, this was an academic paper (or article), and attribution would be obligatory.
When I was in law school I was on Law Review. I remember one student who submitted an article for submission and I was was assigned to check the footnotes.
The student lifted just about every word of the article from various law review articles and legal books without attribution. I pointed out to the Dean that this student had pretty much plagiarized the entire article and when he was called on the carpet he threatened to sue me for slander and libel.
I hope that creep never got his bar ticket. But he probably did.