FreeBSD can bundle it in the installer, and put the root filesystem in a ZFS dataset.
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.
Of course Linux can do this. I'm writing this very reply on a Linux system booted directly off a ZFS dataset.
% zfs list rpool/ROOT/default
NAME USED AVAIL REFER MOUNTPOINT
rpool/ROOT/default 14.8G 60.1G 11.9G /
Support for this exists in GRUB and initramfs-tools, and it works perfectly.
The GPL is not an EULA. It's a distribution licence. There is absolutely no problem in using ZFS on your system, and there's no legal restriction on what an installer may or may not do.
If said installer downloaded and compiled the code (nvidia-drivers style), you'd not have an issue. Only once the code is combined does it "violate" the license. You can't distribute something in violation of GPL2, but you can keep it for yourself. As long as the installer doesn't distribute offending binaries, you're fine.
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.