Richard Stallman of the FSF has a rebuttal to that opinion:
> 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 is 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 this implication. Using a noncopyleft license is weak, and usually an inferior choice, but it's not wrong.
> In other words, selling exceptions permits some embedding in proprietary software, and the X11 license permits even more embedding. If this doesn't make the X11 license unacceptable, it doesn't make selling exceptions unacceptable.
With all due respect to RMS, having read the linked page, that's a weak rebuttal. He seems to reject the premise on the grounds that it's sometimes inconvenient, but accepts it entirely on philosophical grounds when explaining why the FSF doesn't dual-license.
So I think he agrees with my general argument that dual licensing does undermine the principles of free software, but accepts that not everyone can afford to be a zealot.
I'm just suggesting that more appropriate licensing schemes already exist if a developer wants to be paid.
> He seems to reject the premise on the grounds that it's sometimes inconvenient
His argument is that if you consider dual licensing to be immoral because it enables the development of proprietary software, then to be logically consistent, you must also consider non-copyleft FOSS licenses such as BSD/MIT to be immoral because they do the same to an even greater extent. Stallman does not say that dual licensing undermines free software, he only says that it does not support free software as much as copyleft licensing does.
The FSF only uses copyleft licenses for software instead of non-copyleft FOSS licenses or dual licensing, sidestepping this issue entirely. They don't have a problem if people use BSD/MIT, so they also don't have a problem if people use dual licensing.
> 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 is 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 this implication. Using a noncopyleft license is weak, and usually an inferior choice, but it's not wrong.
> In other words, selling exceptions permits some embedding in proprietary software, and the X11 license permits even more embedding. If this doesn't make the X11 license unacceptable, it doesn't make selling exceptions unacceptable.
https://www.fsf.org/blogs/rms/selling-exceptions