The FSF did not add the license to your source code, though. You did that (or another contributor), and the FSF gave you permission to use the text of their license. Adding the license does not assign copyright to the FSF.
> In order to make sure that all of our copyrights can meet the recordkeeping and other requirements of registration, and in order to be able to enforce the GPL most effectively, FSF requires that each author of code incorporated in FSF projects provide a copyright assignment, and, where appropriate, a disclaimer of any work-for-hire ownership claims by the programmer's employer. That way we can be sure that all the code in FSF projects is free code, whose freedom we can most effectively protect, and therefore on which other developers can completely rely.
Forfeiture of copyright would mean it becomes public domain and there are no longer any restrictions on distribution/modification.
If you mean transfer of copyright to an organisation aiming to maximise software freedom for end-users, then Software Freedom Conservancy is willing to receive copyrights and enforce the GPL and other code licenses. Probably the FSF is willing to do that too.
No worries! Yeah, the GPL isn't necessarily as strong as it could be, but that's deliberate: the GPL has a very specific and somewhat limited purpose, and is designed for that purpose only.
The principle beyond "Free Software" (in the FSF/GPL style) is that if you as a consumer receive a bundle of GPL'd software, you as the consumer will also receive the source code to that software. In addition, the GPL as a license is also designed so that if you receive software that partially contains some GPL code, you are given enough additional source code that you can make changes to that code and run your own version of it. For example, if you had a C application and included a single file that you obtained under the GPL, then compile it and distribute it to users, you can't just provide that one GPL file, because the user can't make changes to that file and run it - that's not how compiled C allocations work. You therefore need to provide the source code of the entire application - this is the basis of the viral aspect of the license.
The different versions and variants of the GPL affect how exactly that latter point works, and what forms of including GPL software in non-GPL software is allowed, but generally these are the relevant principles for GPL: if you distribute free software, you distribute its source code as well, and you distribute that source code well enough that the end-user could replace that source code if necessary.
The big difference between GPL and more permissive licenses is really only with the second point - the MIT license already ensures that if you distribute software using MIT-licensed code, you must provide the source code alongside it.
And importantly, all of these ideas apply to the people distributing software - the people running software are fundamentally free to do whatever they want with it. This is why "this code can't be run by ICE" or "this code can only be used for peaceful applications" are not considered free or open source - they aren't about the right to modify code you have obtained, but instead they restrict the right to run that code in the first place. And that's what makes things difficult with these questions around how services like Redis get licensed. If you download an application and its source code, make changes to it, and run it on your servers without distributing it to others, the ideas behind free/open source software say that you should be able to do that. But if you let other people access that running application on your servers, are you distributing that software to them? That's the big open question right now.
