It's not an oversight.
, the Tau don't get them in the Hunter.
The key comes from the fact that the Gladius, lion's Blade, Decurion, Hunter's Cadre, etc, are all made up out of FORMATIONS, and not DETACHMENTS.
In a Formation, there are no open slots. At best, you'll see something like '1-2 Unit X, 0-3 Unit Y or Unit Z' (much like the Demi Company). But you'll never relaly have freedom of choice.
In Detachments, you get a force org chart where you get 1-2 HQ, 2-6 Troop, etc (for example). You need something like this to be able to field a Fortification or a Lord of War, as it is specified in that chart.
And also, no, MS, to my knowledge, there's no option for a Fortification Detachment. That's just how Battlescribe adds it to the party. If that were the case, then there wouldn't be any downside to something like a Decurion, because you could just slot the Fortification detachment in with no fear or repercussions.