Legal documents definitely are copyrightable, and you'll occasionally see enforcement actions. I worked at a firm that represented a major insurance company in a IP/copyright action against another insurance company, claiming that the defendant blatantly copied several of its contracts. (I think our client prevailed.)

That being said, it's often difficult to get past the merger doctrine in copyright claims over legal doctrines. There are only so many ways to express a warranty clause or choice of law provision, so it's usually a strong defense.