You ask a very good question, so my answer will not be so black and white, but I think that this might help you. Note that my response will touch upon the legal issues only and not any ethical issues regarding plagiarism.
A “public domain” work is one that is not protected by copyright law and can therefore be used by anyone without legal consequence. There is a common misconception that just because a work is available online, it qualifies as public domain. In your case, however, it is generally accepted that federal and state legal decisions are in the public domain. Please be mindful that any annotations, headers, or summaries added by commercial publishers like Westlaw are subject to copyright protection, so if any material like that was used, then you would need permission from that commercial publisher.
When one uses a public domain work, the status of that underlying public domain work does not change—it will always be public domain. Your association has copyright protection in your publication’s article as a whole as well as all parts that did not come from the appellate court decision, but your association will be unable to prevent others from excerpting the public domain material from your article.
Answered on Jan 19th, 2016 at 8:02 AM