The other AES implementation is not licensed and thus can not be exported for use with ZFS.

(And no, it's not the USA crypto export restrictions circa 1994, it's two pieces of freely available code with different notions of freedom coded in.)

This is only part of the zfs on linux port, not part of the kernel proper (i.e. mainline). The kernel team is responsible for their part. and the zfs on linux team, since they have to import their own, due to licensing concerns must maintain the library they ship.