TL;DR FreeBSD will rebase its ZFS code from Illumos to ZFS on Linux (ZoL).
If the ZFS FreeBSD code are going to be taken from ZoL, what's the difference of use ZFS on FreeBSD and GNU/Linux?
Because of GPL incompatibility with the CDDL, Linux cannot do this. A compliant installer must put root on something else, usually ext4 or xfs. A Linux installer iso putting root in a zfs dataset opens a terrible legal door.