Earlier quoted context omitted.
FYI, you're right about GNU (by and large), but mistaken about Linux.
GNU has contributor agreements?
https://www.gnu.org/licenses/why-assign.en.html
So while I agree with other commenters that a CLA is a clear indication that the entity seeking to have copyright assigned wants to reserve the right to take some kind of legal action at some point (like changing the license), it also applies in cases where the legal action is benevolent rather than malevolent (like defending the copyright).