Under the GPL, a "combined work" must also be distributed under the GPL. Given that use of certain kernel APIs requires the inclusion of large GPL'd header files, the kernel developers require that any modules that include this GPL'd code be also released under the GPL.
In order to make it more difficult for developers of kernels modules that break the law by running roughshod over the GPL, the kernel developers decided to only make these available to modules that claim to be under the GPL [10]. Please note that module developers can still easily distribute modules that use the "GPL only" APIs but they have to explicitly state that they are using GPL only code (thereby signalling that any copyright infringement is willful).
Calling this "DRM" seems a little disingenuous to me. Anybody can legally remove these protections from their own kernels without any legal repercussions. Removing DRM protections (in the USA) is a felony that carries up to five years in prison. DRM is intentionally obfuscated and poorly documented. The kernel makes its license clear and obvious.
Under DMCA 1201, this sort of measure would probably be legally considered DRM. All you need is a technical mechanism that controls access to a copyrighted work. You do not need obfuscation or encryption in order to make 1201 claims.
Whether or not GPLv2 precludes a DMCA 1201 claim is still in question. On one hand, the GPL says you can make whatever changes you like. On the other, the Linux kernel module loader is specifically enforcing GPL's share-alike requirement, which is a predicate of the right to make modifications to the kernel. This is the heart of what DMCA 1201 is trying to do: extend copyright protection to software that enforces copyright.
If Linux had been re-licensed to GPLv3, this wouldn't be an issue, because v3 specifically has a clause intended to disclaim any and all construable technical protection measures under DMCA 1201.
In order to make it more difficult for developers of kernels modules that break the law by running roughshod over the GPL, the kernel developers decided to only make these available to modules that claim to be under the GPL [10]. Please note that module developers can still easily distribute modules that use the "GPL only" APIs but they have to explicitly state that they are using GPL only code (thereby signalling that any copyright infringement is willful).
Calling this "DRM" seems a little disingenuous to me. Anybody can legally remove these protections from their own kernels without any legal repercussions. Removing DRM protections (in the USA) is a felony that carries up to five years in prison. DRM is intentionally obfuscated and poorly documented. The kernel makes its license clear and obvious.
[0] https://lwn.net/Articles/154602/