What the dogma prevents is from distributing kernels with ZFS built in, this is by default you can't boot of a ZFS partition, but this is hardly a problem when you can modify your initram to have the ZFS loaded on boot from the classic /boot formatted as EXT4.
"Hardly a problem", but now you need to roll your own kernel packages and distribute them to servers
"Hardly a problem", but instead of full disk ZFS and all ZFS fs, we're using an ext /boot like its the 90s again.
"Hardly a problem", just run these 30 simple commands in a console during installation
All of this is because GPL/CDDL license incompatibilities. CDDL code has a per-file copyleft, and. for other operating systems (eg: OS X, Solaris, FreeBSD, illumos etc), the CDDL is perfectly compatible, but GPL considers this too weak to be allowed to be combined in to a GPL work.