That's pretty much the defining factor that makes them militia: They're immobile.
There are various conditions that affect whether or not you can assign militia. It includes battles in the region, the dropping or assigning of other militia units, and probably a few other factors as well. If you really want to assign a unit, then make sure there are no battles in that region, and no other militia-related activities, including reinforcing a unit or dropping a unit, for two weeks. This is the thumbrule I've heard from various generals in the past. I've been assigned a unit once, but never had the desire or opportunity to assign one myself.
