Damn, I had completely missed that in the building description, even though now it's pretty obvious. I just thought those buildings provided auxilliaries-like troops, but otherwise didn't really contribute to cultural conversion unlike the hellenic military outposts. Guess I'll read the descriptions a lot more carefully now.
The 50% unremovable unrest seems a bit high however. It's extremely limiting, especially in the early game. I mean, you're going to need to garrison Marakanda a bit (or a lot) anyway, since you have the Saka and Pahlava to worry about, but right now, you can't even leave it for a few turns with less than 4-5 units and a general.