That means that the contributors agreed to license the code as "GPL version 2, or at your option any later version", which means that they are amenable to changes to the GPL. The FSF could not, however, make a GPLv4 that was a proprietary software license or something. I wouldn't have signed copyright assignment forms if that was a possibility.

Really, FSF copyright is a very good thing, and you can even do it completely digitally in the US and Germany (used to be that you had to use snail mail to receive forms and send them back). People shouldn't get scared away from contributing to GNU projects that choose copyright assignment. There's no better place to protect the GPL than at the FSF.

And that's where I don't fully agree. There are many projects licenced under "GPL v2 or later" and they don't need copyright assignment. Copyright assignment is huge and opens the door for way too many legal stuff.

Also Copyright is a pretty US thing. Law differs and many Countries don't actually allow it. Also since you brought up Getmany already: https://en.wikipedia.org/wiki/Copyright_law_of_Germany#Trans... Germany is such a country and even if you were to assign copyright to them it would be legally void to the best of my knowledge. And also other countries that base it on moral rights.

Copyright law is very different between countries and way too strong to just transfer for such a use. If they really wanted to, they should have made a license to handle it exactly the way they wanted.

The problem is that in order to have the ability to register their copyright and do meaningful enforcement actions in the US, they need copyright assignment. Copyright cares about authorship, so if they don't have "authorship" (read: own the copyright) then they have very little power to enforce the GPL.

Some other people argue it's not necessary, and while that might be true, it's better to be safe than sorry. Not to mention that all GNU projects are GPLv3-or-later anyway (so any contribution would be able to be upgraded anyway since it's licensed in such a way that you can use it under GPLv4). If you refuse and license your change under GPLv3, it won't get merged.

It says:

5. FSF agrees that any program "based on the Works" offered to the public by FSF or its agents or assignees shall be offered in the form of machine-readable source code, in addition to any other forms of FSF's choosing. However, FSF is free to choose at its convenience the media of distribution for machine-readable source code and may charge a fee of its choosing for copies.

“similar in spirit” is a pretty strong condition, since there are lots of philosophy texts on the GNU pages which make it clear what “similar in spirit” means. A court would likely rely on these to decide a case.