There are actually two possibilities here:
(1) The text of the GPLv2 does legitimately restrict user programs as derivative works of the kernel, and therefore a license exception is needed from the licensors in order avoid violating the license or copyright law.
(2) User programs are not legitimately derivative works of the kernel, and therefore a license exception is not required, and anything said on the subject is mere commentary about the prevailing legal state of affairs.
In my opinion, the idea that user programs are - in general - derivative works of the kernel is nonsense, but for people who worry that they are or might be, the clarification or exception (whichever) is quite convenient. A court is not going to enforce a requirement on the licensees that the licensors and copyright holders explicitly deny and disavow, no matter what any third party thinks about the subject.
The situation with in kernel modules is more complicated, because there the licensor copyright holders appear to be convinced that nearly any kernel module is legitimately classified as a derivative work, so estoppel doesn't apply and whether a kernel module is actually a derivative work depends such things as how much kernel inline code was included in the compilation process, if not other apparent legal nonsense like compatibility and purpose which appears to have no basis in copyright law as something that in and of itself makes for a derivative work at all.