It's better / more accurate to see this change as "OpenZFS migrating to ZFS-on-LInux" rather than "FreeBSD moving to ZoL".
Currently, the illumos source tree for ZFS is the "canonical" upstream source repo for OpenZFS. Other projects take that as the starting point, do their own developments, and then request their features be added to the illumos-gate repo. However, there's a lot of politics and waiting around to get those changes into illumos-gate. Meanwhile, there's lot of new development happening outside of illumos-gate that's not getting pushed back upstream, and not flowing back downstream to others. This is leading to a collection of not-always-compatible implementations of ZFS on different OSes. (People complain about Android fragmentation all the time ... well, this is starting to happen with ZFS.)
So, the OpenZFS folks are looking to rebase the "canonical" upstream source repo on the ZoL sources. Then split that OpenZFS repo into "OS independent" and "OS dependent" directories for each OS (FreeBSD, Linux, Delphix, Illumos, Windows, MacOS X, etc). And moving to a more open / standardised way for developing features in the downstream OS repos and pushing those changes upstream, in order for other projects to get access to them.
This will still be done under the OpenZFS umbrella. It's still the same set of developers working on things. And there's work underway to get the FreeBSD-specific features (like TRIM support and NFSv4 ACLs) into the ZoL tree. Nothing will be changing in the FreeBSD releases until there's feature and stability parity between "old" ZFS and "new" ZFS (which won't happen until at least 13.0).
So, there's no reason to get your knickers all in a twist.
For more info on this, check out the
OpenZFS website, the Monthly
ZFS Leadership Meeting minutes and
videos, and the
Feature Ownership spreadsheet. It's an exciting time to be a ZFS user, as there's a lot of nifty features coming down the pike. Features that would take years to get into FreeBSD if we had to wait around for things to get into illumos-gate first.