Plagiarism and copyright infringement are two separate issues.
BTW, some legal documents are copyrighted. I have seen several US patent applications with copyright notices affixed. Specific phrasing, technical drawings, maskworks, and expressions can be construed as artistic works even though they serve a substantive purpose.
You can be guilty of plagiarism of non-copyrighted texts. Plagiarism is not dependent on copyright. You can plagiarize from public domain works like Shakespeare (or a court pleading) and it is still plagiarism.
It is unlikely that a state bar would disbar of even discipline an attorney solely because of claims of plagiarism. Not impossible, but very, very unlikely because of the nature of the profession most of the members of the disciplinary board will have plagiarized plenty in their careers.
