I still occasionally hear someone say that if you violate a General Public License, then you can be forced to give away all your own code. This is false.
In the US, all computer code is protected by copyright law, unless you can find code older than Mickey Mouse. Someone owns that copyright. Just because you find code on the internet does not mean that you have permission to redistribute it. Even code for Microsoft's Windows has appeared on the internet from time to time.
You only have permission to redistribute copyrighted code if you get permission from the copyright owner, who by default is the author of that code. The law is no different here than for any other form of written text.
A license grants permission under certain circumstances. If you follow the terms of that license, then you can use the code accordingly. If not, then you have violated a copyright, and the license is irrelevant. You can be sued for damages the same as if there were no license at all.
The GPL is not a contract. The GPL matters only if you think you have followed the terms of the license. If you do not follow the license, then you are admitting you need some other form of permission to redistribute the code.
As the GPL states "You are not required to accept this License, since you have not signed it. However, nothing else grants you permission to modify or distribute the Program or its derivative works. These actions are prohibited by law if you do not accept this License."
Let me emphasize: the GPL grants permissions and rights; copyright law restricts them, by default.
If the copyright owner finds that you have redistributed his code without permission, he has many choices. He can ignore the problem, grant permission anyway, offer an alternative license, sue for damages, or negotiate with you for a settlement. The situation is no different than if there were no GPL.
One of the offered settlements may be to follow the GPL, such as releasing your modifications and extensions of the code. If you refuse, then the copyright holder has no way to force you to follow the GPL terms. You are, however, using stolen code without permission. Don't be surprised to be sued for damages.
So far as the GPL is concerned, these issues have been well-explained since the 80's. See frequently asked questions about the GPL at http://www.gnu.org/licenses/gpl-faq.html
Bill Harlan, 2007
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