Selling Exceptions to the GNU GPL
Selling exceptions means that the copyright holder of the code releases it to the general public under a valid free software license, then separately offers users the option of paying for permission to use the same code under different terms, for instance terms allowing its inclusion in proprietary applications.
We must distinguish the practice of selling exceptions from something crucially different: purely proprietary extensions or versions of a free program. These two activities, even if practiced simultaneously by one company, are different issues. In selling exceptions, the same code that the exception applies to is available to the general public as free software. An extension or a modified version that is only available under a proprietary license is proprietary software, pure and simple, and just as wrong as any other proprietary software. This article is concerned with cases that involve strictly and only the sale of exceptions.
We must also distinguish selling exceptions from dual licensing, which means releasing the program under a choice of licenses. With dual licensing, each user can choose to use the program under either one of the licenses, or under both in parallel for activities that fit both. (Thus, redistributors normally pass along both of the licenses.) For instance, Perl was distributed for many years under a dual license whose alternatives were the GNU GPL and the Artistic License. That is not necessary any more because version 2 of the Artistic License is compatible with the GNU GPL.
In selling exceptions, the exception's terms are not a second license that the program is released under. Rather, they are available only to those users that buy an exception. The only license that the release carries is the GNU GPL, so this is not dual licensing.
We must distinguish selling of exceptions from the usual kind of “exception to the GPL,” which simply gives all users permission to go beyond the GPL's conditions in some specific way. These exceptions are governed by section 7 of the GNU GPL. Selling exceptions is legally independent of the GNU GPL. To avoid confusion it is best not to refer to exceptions that are sold as “exceptions to the GPL.”
I've considered selling exceptions acceptable since the 1990s, and on occasion I've suggested it to companies. Sometimes this approach has made it possible for important programs to become free software.
The KDE desktop was developed in the 90s based on the Qt library. Qt was proprietary software, and TrollTech charged for permission to embed it in proprietary applications. TrollTech allowed gratis use of Qt in free applications, but this did not make it free/libre software. Completely free operating systems therefore could not include Qt, so they could not use KDE either.
In 1998, the management of TrollTech recognized that they could make Qt free software and continue charging for permission to embed it in proprietary software. I do not recall whether the suggestion came from me, but I certainly was happy to see the change, which made it possible to use Qt and thus KDE in the free software world.
Initially, they used their own license, the Q Public License (QPL)—quite restrictive as free software licenses go, and incompatible with the GNU GPL. Later they switched to the GNU GPL; I think I had explained to them that it would work for the purpose.
Selling exceptions depends fundamentally on using a copyleft license, such as the GNU GPL, for the free software release. A copyleft license permits embedding in a larger program only if the whole combined program is released under that license; this is how it ensures extended versions will also be free. Thus, users that want to make the combined program proprietary need special permission. Only the copyright holder can grant that, and selling exceptions is one style of doing so. Someone else, who received the code under the GNU GPL or another copyleft license, cannot grant an exception.
When I first heard of the practice of selling exceptions, I asked myself whether the practice is ethical. If someone buys an exception to embed a program in a larger proprietary program, he's doing something wrong (namely, making proprietary software). Does it follow that the developer that sold the exception is doing something wrong too?
If that implication were valid, it would also apply to releasing the same program under a noncopyleft free software license, such as the X11 license. That also permits such embedding. So either we have to conclude that it's wrong to release anything under the X11 license—a conclusion I find unacceptably extreme—or reject the implication. Using a noncopyleft license is weak, and usually an inferior choice, but it's not wrong.
In other words, selling exceptions permits limited embedding of the code in proprietary software, but the X11 license goes even further, permitting unlimited use of the code (and modified versions of it) in proprietary software. If this doesn't make the X11 license unacceptable, it doesn't make selling exceptions unacceptable.
There are three reasons why the FSF doesn't practice selling exceptions. One is that it doesn't lead to the FSF's goal: assuring freedom for each user of our software. That's what we wrote the GNU GPL for, and the way to achieve this most thoroughly is to release under GPL version 3-or-later and not allow embedding in proprietary software. Selling exceptions wouldn't achieve this, just as release under the X11 license wouldn't. So normally we don't do either of those things: we release under the GPL only.
Another reason we release only under the GPL is so as not to permit proprietary extensions that would present practical advantages over our free programs. Users for whom freedom is not a value might choose those non-free versions rather than the free programs they are based on—and lose their freedom. We don't want to encourage that.
There are occasional cases where, for specific reasons of strategy, we decide that using a more permissive license on a certain program is better for the cause of freedom. In those cases, we release the program to everyone under that permissive license.
This is because of another ethical principle that the FSF follows: to treat all users the same. An idealistic campaign for freedom should not discriminate, so the FSF is committed to giving the same license to all users. The FSF never sells exceptions; whatever license or licenses we release a program under, that is available to everyone.
But we need not insist that companies follow that principle. I consider selling exceptions an acceptable thing for a company to do, and I will suggest it where appropriate as a way to get programs freed.