"ZFS is free software, but there is a GPL incompatibility that keeps it out
of the kernel tree."
Just like a GPLv3 kernel module would be kept out. v2 and v3 are not
compatible with each other, without the v2 stuff being upgraded to v3 via the
"any later version clause." The kernel lacks that so it, too, would amount to
a violation. You could still say that GPLv3 kernel module is free software
but it doesn't change that the combination is still not allowed (and who does
the linking doesn't matter, as the FSF explained in their article. So
Trisquel can't get around that by saying "Oh, the user did it.") The
incompatibility keeps it out of kernel.org and also stop Trisquel (and
others) from being able to distribute it legally.
"If you remove the ZFS packages (which would require the user to take action
to install locally for private use), then that means you should remove all
other free software from the repositories if it butts heads with the GPL. If
you don't it makes you look silly and biased."
Not all GPL-incompatible programs form a modified or extended version of the
kernel. ZFS does, whether you personally believe it or not. Trisquel can't
legally distribute it without running afoul of both the GPL and CDDL. (The
Software Freedom Conservancy explained why it's a violation of both licenses
simultaneously.) But I don't expect you to believe them either.
