Comment by GuB-42

It is a problem because most of the internal kernel APIs are GPL-only, which limit the abilities of the ZFS module. It is a common source of argument between the Linux guys and the ZFS on Linux guys.

The reason for this is not just to piss off non-GPL module developers. GPL-only internal APIs are subject to change without notice, even more so than the rest of the kernel. And because the licence may not allow the Linux kernel developers to make the necessary changes to the module when it happens, there is a good chance it breaks without warning.

And even with that, all internal APIs may change, it is just a bit less likely than for the GPL-only ones, and because ZFS on Linux is a separate module, there is no guarantee for it to not break with successive Linux versions, in fact, it is more like a guarantee that it will break.

Linux is proudly monolithic, and as constantly evolving a monolithic kernel, developers need to have control over the entire project. It is also community-driven. Combined, you need rules to have the community work together, or everything will break down, and that's what the GPL is for.