Yes it is plagiarism, but it’s perfectly legal.
Sometimes I will come across a case which I am not allowed to quote because it is deemed an “unpublished opinion.” If I think the arguments are germane to my case, I will literally steal the language of the opinion and put it in my brief without attribution.
Technically I could be sanctioned if I actually referenced where I stole the language from.
Per court rules you can’t reference an opinion that was not “certified for publication” (even though it is published) but you can literally steal the entire opinion and put in in your own brief as if they are your own words.
It’s stupid, but it’s the law.
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.
There are many things that are legal but are immoral, unfortunately. Abortion comes to mind.