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