It's part of Microsoft's certification requirements, *for x86 systems*. The requirements for ARM were the exact opposite (they say that the user must *not* be able to change the key), though since barely anyone's doing Windows-on-ARM any more that's becoming increasingly less relevant. The UEFI specification (where Secure Boot is actually defined) doesn't prescribe anything about how it should be set up out of the box in any particular firmware implementation (whether any keys should be pre-loaded, whose they should be if so, whether there should be an interface for changing them, etc.)