Theoretically, nvidia.ko never comes in the same dvd / usb-stick / download as the kernel, it's always a separate piece installed by the user later (even if with a helper program). "Super Easy" derivatives of mainstream distros may include nvidia.ko (and libdvdcss etc) in the initial install, but mainstream distros including ubuntu do not. So they could say, if there was some violation of license going on, it wasn't them doing it, because they were not distributing something derived from the linux kernel. They may separately offer some "shim" code and binary under a separate license that the user then combines with the linux kernel headers on their own machine, and does not distribute the combination.
That's one of the attempted justifications anyway. So what's more "audacious" here is Ubuntu distributing zfs.ko included in the initial install, and not requiring a dkms-like setup.
There's disagreement, even among core kernel developers, about whether nvidia's binary kernel module is OK. The fact that they've gotten away with it for a long time doesn't mean that it's OK, just that it's somewhere between hard and impossible to enforce copyleft (for various reasons worth of a separate post). ref: https://lkml.org/lkml/2012/8/1/411
Then, theoretically, how about distributing the zfs-on-linux source code with the NAS and present a button in the web interface "Install ZFS" which pops up a big "this will install a non-GPL-compliant module" message which everyone just clicks "OK" on and we're done?
This is essentially what DKMS does and how Debian is shipping ZFS. I can only imagine that Ubuntu wants to make it easier that (perhaps it would be hard to have the installer support ZFS as root filesystem without having the binary ZFS module - you'd have to ship a full compiler toolchain as part of the installer?)
A copyright holder for the linux kernel, on the grounds that they are distributing an unlicensed derivative work thereof (there would be no need to show damages, there are statutory damages; it's definitely unlicensed, the part that's not clear is whether it's a derivative work). It's not really in the EFF's wheelhouse; mjg59 (who holds at least some kernel copyrights) claimed to be talking to the software freedom law center about his options. I'm unsure why he'd regard zfs.ko as more problematic than nvidia.ko (I mean fundamentally end users have the access they need to fix bugs in the ZFS source, the CDDL is basically the same as the GPL, whereas end users cannot fix bugs in the NVidia video drivers, not in some theoretical legal sense but in actual practice), and if the legal theory is correct then it surely applies equally well to both, but he's the one with standing to sue so it's his call. IANAL.