Interesting that it took so long to reach this conclusion. I thought Linux devs stayed away from ZFS mainly due to the licensing which leads me to believe it's not quite as 'obvious' as this post mentions.
Linux stayed away from including it into the shipped kernel, which is indeed a derivative work. Their argument is that kernel module loaded into kernel separately is acceptable to avoid GPL violation.
If this is acceptable then the GPL is useless. This behavior is not in the spirit of the GPL and copyleft.
GPL defines what is supposed to be the subject to its limitations. And apparently this use case isn't part of it. So how do you define what's in spirit and what is in the letter of the law here?