Robbie at Reddit wrote this a while ago:
What is with all these files being hidden lately? It's ridiculous. You always had the odd author who deleted their work for some reason or another, but it's to the point where we're losing major mods that he community loves all the time. It seems to have gotten way worse over the last year or so, and these threads are popping up all the time. I don't understand why the authors don't just say "hey I'm done updating this and will no longer be offering support" if they don't want to be involved anymore.
Robbie is referring to a string of "mod closures" affecting TES V: Skyrim, where mature mods are being permanently removed from hosting platforms for reasons generally personal to the author. These are mods that have been downloaded thousands and sometimes tens of thousands of times by users across the globe, only to suddenly disappear and often without explanation. In most cases these authors are exercising their creative rights (administered by the Skyrim EULA). A couple well-known examples are Wyrmstooth and Dragon Combat Overhaul (DCO). A compilation of examples can be found here.
On one end, it is within the authors' domains to remove their mods from a hosting platform such as Nexus. Nexus does not claim rights to anything; the resources of the mod are distributed at the discretion of the author. But on the other hand, Bethesda's license restricts commercialization through any venue outside of their own. This means that- aside for any resources such as art or sounds - the author doesn't really have a claim to their intellectual property that would hold up in court. For example, suppose a user hosts another user's mod after they disappear. Since there's absolutely no way for any party to make money off of this venture (except, maybe, via advertising), it would be difficult for the author to prove to a court that he or she has suffered damages.
The point here is that a textbook case in favor of open-sourcing mods can easily be made. Since a mod cannot be commercialized by an author, there's no opportunity cost associated with a reticent author who keeps his/her Papyrus code and assets hidden from the community. There is simply no economic reason to hide a mod. It would of course be different if an author could sell his mod with Bethesda out of the picture. Then, one could certainly make a case that an author would want to keep the work under wraps as to have an option of commercializing without issue.
Sadly economic reasons are not the prime motivator for keeping mods hidden. Generally Skyrim authors want to maintain their status and do not want their hard work copied and re-hosted, which is a real risk in a gaming community. It has and continues to happen. This is actually very reasonable; since there's no money to be made, all an author has is reputation. And reputation is quickly damaged when one's work is stolen and others pose as the authors. Such actions cause confusion in the community and bite into the author's fan base. This is often a demoralizing occurrence that makes mod authors less likely to continue their work with enthusiasm.
But let's back up a bit. What is open-source? Open-source means precisely that anyone can edit publicly available open-source code. Open-source licenses place no restrictions on forks of projects, but some posit certain obligations. Open source licenses universally allow free distribution of the source code. Where the licenses differ is in attribution and obligation. The most 'restrictive' open-source licenses require future authors to contribute back into some master repository should they modify or enhance the code, or to attach the same license to any new code that is produced.
Code that is freely distributable, but not open-source is impractical. In reality there are rarely if ever such projects. This is because for code to be that way it must have been acquired publicly, and to be considered freely distributable in the eyes of a court it must have a license clearly dictating these terms. It would be quite something else to see a license where only the original author is permitted to distribute the source code. Continuing in the same vein:
For code to be open-source it has exactly two requirements at minimum:
Publicly available source-code
Source-code is covered by an open-source license in its entirety
Source code that is not publicly available is known as closed source. It could be unlicensed, or even have an open-source license. If it cannot be reached by total strangers using the internet it is considered closed. By definition, closed code cannot be open-source, regardless of the license.
Source code that is not publicly available, and covered by a non-open source license (such as a commercial license) is known as proprietary code. Should you host such code publicly, it would be considered either malicious or accidental. Such code, because of its restrictive license, cannot be used in other projects. Most companies and government operations will have a large amount of proprietary code. Such proprietary licenses generally stipulate that if the code is used for any purpose outside of the company it is subject to lawsuit.
Code that is proprietary (licensed to a particular company or individual) and available publicly is considered a mistake or an error. Furthermore, code that is unlicensed and publicly available is actually not open-source!
Code that is publicly available but has no license is known as unlicensed. Because it has no terms dictating its use, it is generally not safe to use in either commercial or open-source projects. Unlicensed code cannot be open-source by definition. This kind of code has actually been a huge problem for hosting platforms like GitHub. More recently GitHub has streamlined the process of assigning a license to a project. In contrast, proprietary code in most circumstances is stored privately and covered by a restrictive license.
There are many, many open-source licenses. As mentioned above, they universally share one trait: the code they cover is freely usable and re-distributable by all individuals and entities. They differ in levels of restrictivity. But this may be the wrong word- anyone and everyone can still use any open-source code. But more restrictive open source licenses present more obligations to the users of the code. Some require attribution (perhaps to an individual or even a company) when the code is used in a project. Attribution could mean presenting a logo in the application. Other licenses like GPL obligate any users to contribute their changes to the source back to some repository or the general community, and to maintain the same license in any new code. Here's a quick definition from opensource.org:
Open source licenses are licenses that comply with the Open Source Definition — in brief, they allow software to be freely used, modified, and shared.
My rant here is probably going to continue again at a later date, discussing some sort of middle-ground between open mods and closed mods. The point to remember is that if a mod author's work was open-source, the community could pick up where they left off should they disappear. But since often their work is either closed or proprietary, the community is unable to continue to support a mod without significantly more effort. This, unfortunately, makes it all the more painful to the community when authors take their mods down and disappear.