Fortunately for you, most of you have never been in law school (I like to tell prospective law students that there are more amusing prisons in the mountains of Peru.). So you have had no reason to look at a law school casebook.
The name casebook distinguishes them from textbooks used by students in other fields. A casebook consists mostly of the reports of the decisions of courts (most often appellate courts) in actual decided cases. The case reports (thus the name cases) are usually edited to remove material not relevant to the main point the author is interested in (often irrelevant and trivial procedural issues from the lower court), the casebook author often includes notes of contrary or different cases and of relevant statutes.
There is usually no other material created by the so-called authors of the casebook.
Here is the main point. Reports of cases decided by courts are inherently public domain material regardless of their age. The same is true of statutes.
Almost all of the material in a casebook is not, and may not be, copyrighted.
Asserting intellectual property rights over reprinted public domain material is requires nerve to the point of chutzpah.
BTW, the restriction on resale is mostly like invalid as a restraint of trade, and will not be binding on subsequent purchasers such as used book merchants. It is also a violation of the Sherman Act.