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.