It isn't so much that Copyright Law is unclear - the problem is that the law changes over time and the law applies to publications written while a particular version of the law was in effect.
Most Watchtower publications printed before 1970 were printed WITHOUT a copyright notice and under the law then in effect, those publications went automatically into the Public Domain. I believe this apparent "oversight" on the part of the Watchtower was likely due to the Watchtower believing their own "end of the world" hogwash.
After approximately 1970, the Society "lawyered up" and began looking to protect their vulnerable areas - and NOTHING hurts the WTB&TS more than their old publications.
The current copyright law was altered by Senator Sonny Bono and others in the entertainment industry to provide much longer protection (about a century, if memory serves). The WTB&TS is VERY happy to take advantage of Caesar's protection.
Many apostates believe that information about the WTB&TS should be free, so they take advantage of the copyright law's "fair use" provision and sometimes defy the law in the interest of human freedom.