Yes, the proprietary drivers have always been an ethical and legal grey area. However, the way a proprietary driver is distributed is different to how ZFS is distributed. Proprietary drivers are distributed as object code that is then built to be a kernel module on the user's machine. This means that at no point does Nvidia or AMD distribute a Linux kernel module with proprietary code. There are arguments however that distributions which distribute this auto-build scheme by default may also be in violation of the GPL. ZFS is distributed by Canonical as a fully functional kernel module.
There's a lot of gray areas because there have not been many legal cases on derived works and the GPL. The GPL itself has held up in court on copyleft grounds, of course.
Also, you've got the fact that in the case of proprietary graphics drivers, the threat is that the Linux kernel community would sue Nvidia or AMD. The threat with ZFS is that Oracle (who is a member of the Linux kernel community) would sue people using ZFS (they could also then sue for patent infringement).
I know in the past NixOS did the same for the ZFS kernel module: prior to NixOS installation, it was able to download the ZFS source code, build it and then load it from inside the installer Live CD/USB while it was running, and this would require just a couple of commands. I don't know if this is still true nowadays or if they just distribute the ZFS kernel module directly in the Live installer.
If combining gpl and cddl in a redistributed file is a violation of gpl but not cddl, then what would the holder of the copyright of the cddl code be able to sue about? I think the concern is that a contributor to Linux might sue, just like in the Nvidia/and case.
Oracle is a contributor to Linux, so they could sue from the GPL side. While Nvidia and AMD also are Linux contributors, the fact they ship proprietary modules would make it hard to argue that they aren't implicitly permitting users to redistribute it (and thus they would be forced to license their drivers under GPLv2).
Not to mention that ZFS is covered by Oracle patents. CDDL provides a patent license, but it might be possible for Oracle to sue you for patent infringement if you're distributing code in a way that complicates the licensing. Not that I'm saying that's likely, but Oracle has enough money to ruin you if they want to.
There's a lot of gray areas because there have not been many legal cases on derived works and the GPL. The GPL itself has held up in court on copyleft grounds, of course.
Also, you've got the fact that in the case of proprietary graphics drivers, the threat is that the Linux kernel community would sue Nvidia or AMD. The threat with ZFS is that Oracle (who is a member of the Linux kernel community) would sue people using ZFS (they could also then sue for patent infringement).