You can't be someone who loses their mind over GPL violations, but then willfully misinterpret someone else's open source license because they have something you want. The developers involved specifically picked CDDL because it wasn't GPL compatible. That was their desire / intent, and so that's the license they went with. By ignoring the license and the clearly known intent of the developers in choosing that license, simply because they made something you really want, I believe that ZFS on Linux is really a form of theft in the open source world. It's the same sort of attitude that leads to GPL violations.
I don't agree with making things CDDL to be explicitly GPL incompatible, but I do believe in respecting the author of the software in question and the license he or she chose. If people choose licenses I don't agree with, I don't use their software. I don't just use it anyway and pretend like there's no license issue.
You don't have to be RMS to see that these people who are ignoring the licensing issues (not just shipping user mode tools like a FUSE implementation which is legit) honestly don't care about open source and licensing and copyleft. They just see something someone else made that they want and they take it and ignore the licensing and developer intent. How can we hope to enforce the GPL successfully but then ignore other OSI approved licenses when it suits us?
This is why big names in Linux kernel development aren't remotely interested in ZFS.
Not to mention the fact that the users of ZFS on Linux are all setting themselves up for Oracle to bring the licensing inquisition.