An explanation of how “copyleft” licensing works

This is based on a post on reddit, published on 2013-02-21.

While reading the man page for readline I noticed the copyright section said “Readline is Copyright (C) 1989-2011 Free Software Foundation Inc”. How can software be both licensed under GNU and copyrighted to a single group? It was my understanding that once code became free it didn’t belong to any particular group or individual.

[LiveCode is GPLv3, but also sells non-free licenses] Can you really have the same code under two conflicting licenses? Once licensed under GPL3 wouldn’t they too be required to adhere to its rules?

I believe that GNU/the FSF has an FAQ that addresses this, but I can’t find it, so here we go.

Glossary:

• “Copyright” is the right to control how copies are made of something.
• Something for which no one holds the copyright is in the “public domain”, because anyone (“the public”) is allowed to do anything with it.
• A “license” is basically a legal document that says “I promise not to sue you if make copies in these specific ways.”
• A “non-free” license basically says “There are no conditions under which you can make copies that I won’t sue you.”
• A “permissive” (type of free) license basically says “You can do whatever you want, BUT have to give me credit”, and is very similar to the public domain. If the copyright holder didn’t have the copyright, they couldn’t sue you to make sure that you gave them credit, and nobody would have to give them credit.
• A “copyleft” (type of free) license basically says, “You can do whatever you want, BUT anyone who gets a copy from you has to be able to do whatever they want too.” If the copyright holder didn’t have the copyright, they couldn’t sue you to make sure that you gave the source to people go got it from you, and non-free versions of these programs would start to exist.

Specific questions:

Readline: The GNU GPL is a copyleft license. If you make a modified version of Readline, and give it to others without letting them have the source code, the FSF will sue you. They can do this because they have the copyright on Readline, and in the GNU GPL (the license they used) it only says that they won’t sue you if you distribute the source with the modified version. If they didn’t have the copyright, they couldn’t sue you, and the GNU GPL would be worthless.

LiveCode: The copyright holder for something is not required to obey the license—the license is only a promise not to sue you; of course they won’t sue themselves. They can also offer different terms to different people. They can tell most people “I won’t sue you as long as you share the source,” but if someone gave them a little money, they might say, “I also promise not sue sue this guy, even if he doesn’t give out the source.”