It’s simple reality (and good business): lawyers and judges cut and paste from one another’s documents every day, and it would be absurd to impose a regime of copyright designed to promote original creativity if it undercut a legal regime designed to promote effective representation. Similarly, it is absurd to accuse lawyers of “plagiarism” in the documents they produce as lawyers. There have been cases which have spoken in terms of lawyer plagiarism, but they all can be better understood as fraud (charging clients for research that was nothing but the copying of pre-existing work) or malpractice (excessive copying that produces a document that bears little relationship to what the specific representation demands, as opposed to effective cutting and pasting of pre-existing work into newly written work) than as plagiarism.
Plagiarism is passing off someone else’s work as your own. But, again, in legal practice (as opposed to legal scholarship or law school work), the point of the work is its effectiveness, not its source or its originality.
In other words, all you students of mine, plagiarism in school is not allowed. It is an unethical act that can and will result in expulsion and disqualification from the practice of law. But let’s not confuse contexts: some contexts, specifically academic practices, produce in their audiences the expectation of originality; in others, specifically legal practice, originality can be effective, but effectiveness is the bottom line.