As upstream supplies systemd unit files with sys-fs/udev and udev itself is an important dependency of systemd, its ebuild should use the systemd eclass to enable installing those units.
I am encouraging ebuild authors to migrate to autotools-utils and use systemd_to_econfargs() which provides the eclass with more flexability. If you're interested, I'm willing to provide a patch for such migration. If not, there's still "$(systemd_with_unitdir)".
Fixed in udev-171.