As such, it's very possible for a source tree to have a copy of the GPl that includes such language, but where the person using that source tree does not have to follow the GPL because they have some other license.
This is why the GPL is not 'viral' in the sense that the FUD being spread around implies. Putting a GPL file into your proprietary code does not automatically make your entire codebase GPL, it just means that you have no valid way to distribute that codebase. When you get called on it by a GPL license holder, you have three choices.
1. continue to distribute the codebase, but comply with the GPL
2. negotiate some other license to distribute the codebase.
3. remove the GPL code from your codebase
granting an additional license actually happens quite frequently in the FOSS world. Not in cases where someone has started legal action against a GPL infringer, but it's very common to see someone ask if a particular piece of code can be dual licensed (frequently under BSD) so that some other project with a non-GPL license can make use of it. If the function is fairly small, the reaction is commonly "sure"
If you take option #3, the version you are distributing now is still infringing, and the license holder can still get an injunction and go after you for damages for the version you are distributing, but if you release a new version without the GPL code (and without anything "derived" from that code, which is a slippery definition), then the GPL license holder has no claim against your new version.
In this case, since Twin Peaks was sloppy enough to use one piece of GPL code like this, the odds are really good that they have used more. Every copyright holder of such code can now go after Twin Peaks, and they can each do so individually.
Doing so is a nuclear option that could scare a lot of people away from FOSS code. The incident with Cisco that Rob Landley has been so upset about is a mild example of this. If you were to have 500 individuals sue one company for copyright infringement for the Linux kernel as separate lawsuits (nothing says they need to band together and jointly hire a lawyer), the mere cost of lawyers to handle so many separate cases would be devastating.
But if companies want to go nuclear against FOSS on patents because they don't think FOSS can defend themselves, they need to be prepared to have FOSS fight back with the tools that are available, namely copyrights.
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