Pretty standard really. You don't want contributions to the codebase come under questionable copyright concerns, or the original creator to revoke the code 4 years later causing huge headaches potentially.

You typically have to sign these types of CLA's whenever you need to contribute to any serious project. I've had to do it for Google and Microsoft recently, and I've done it for various other open source projects as well.

Still that shouldn't concern users/gamedevs as they don't contribute to the engine code typically. Only if they want to upstream changes back into the engine publicly they would need to sign it ofcourse