> Makes me wonder what these are for (copyright transfer) and why they decided it’s not needed. It also makes me wonder if this sort of thing has ever been taken/tested in court or if it’s paranoid friction with little value add.

Some companies/projects might use them purely to avoid possible future legal headaches (I think GNU does this), and I'm not sure to what degree that has actually been tested, but they can also allow re-licensing under a different license which is more clear cut and I think that's more the issue here

Amazon is trying to say that they'll never relicense the code, so they have no need to take ownership over contributions.