When a work is made for doing a practical job, the users must have control over the job, so they need to have control over the work. This applies to software, and to educational works too. For the users to have this control, they need certain freedoms (see gnu.org), and we say the work is "free" (or "libre", to emphasize we are not talking about price). For works that might be used in commercial contexts, the requisite freedom includes commercial use, redistribution and modification.
Creative Commons publishes six principal licenses. Two are free/libre licenses: the Sharealike license CC-BY-SA is a free/libre license with copyleft, and the Attribution license (CC-BY) is a free/libre license without copyleft. The other four are nonfree, either because they don't allow modification (ND, Noderivs) or because they don't allow commercial use (NC, Nocommercial).
In my view, nonfree licenses are ok for works of art/entertainment, or that present personal viewpoints (such as this article itself). Those works aren't meant for doing a practical job, so the argument about the users' control does not apply. Thus, I do not object if they are published with the CC-BY-NC-ND license, which allows only noncommercial redistribution of exact copies.
Use of this license for a work does not mean that you can't possibly publish that work commercially or with modifications. The license doesn't give permission for that, but you could ask the copyright holder for permission, perhaps offering a quid pro quo, and you might get it. It isn't automatic, but it isn't impossible.
However, two of the nonfree CC licenses lead to the creation of works that can't in practice be published commercially, because there is no feasible way to ask for permission. These are CC-BY-NC and CC-BY-NC-SA, the two CC licenses that permit modification but not commercial use.
The problem arises because, with the Internet, people can easily (and lawfully) pile one noncommercial modification on another. Over decades this will result in works with contributions from hundreds or even thousands of people.
What happens if you would like to use one of those works commercially? How could you get permission? You'd have to ask all the substantial copyright holders. Some of them might have contributed years before and be impossible to find. Some might have contributed decades before, and might well be dead, but their copyrights won't have died with them. You'd have to find and ask their heirs, supposing it is possible to identify those. In general, it will be impossible to clear copyright on the works that these licenses invite people to make.
This is a form of the well-known "orphan works" problem, except exponentially worse; when combining works that had many contributors, the resulting work can be orphaned many times over before it is born.
To eliminate this problem would require a mechanism that involves asking _someone_ for permission (otherwise the NC condition turns into a nullity), but doesn't require asking _all the contributors_ for permission. It is easy to imagine such mechanisms; the hard part is to convince the community that one such mechanisms is fair and reach a consensus to accept it.
I hope that can be done, but the CC-BY-NC and CC-BY-NC-SA licenses, as they are today, should be avoided.
Unfortunately, one of them is used quite a lot. CC-BY-NC-SA, which allows noncommercial publication of modified versions under the same license, has become the fashion for online educational works. MIT's "Open Courseware" got it stared, and many other schools followed MIT down the wrong path. Whereas in software "open source" means "probably free, but I don't dare talk about it so you'll have to check for yourself," in many online education projects "open" means "nonfree for sure".
Even if the problem with CC-BY-NC-SA and CC-BY-NC is fixed, they still won't be the right way to release educational works meant for doing practical jobs. The users of these works, teachers and students, must have control over the works, and that requires making them free. I urge Creative Commons to state that works meant for practical jobs, including educational resources and reference works as well as software, should be released under free/libre licenses only.
Educators, and all those who wish to contribute to on-line educational works: please do not to let your work be made non-free. Offer your assistance and text to educational works that carry free/libre licenses, preferably copyleft licenses so that all versions of the work must respect teachers' and students' freedom. Then invite educational activities to use and redistribute these works on that freedom-respecting basis, if they will. Together we can make education a domain of freedom.
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