Rules v. 1.01 states that "factories cannot be moved p.3" (although there is a card that lets you do this)
I think you mean "fortresses". In any case, there is only one fortress without a given location (German one). I see your point as we don't want built fortresses being SMd around the map. But in this case of this one unit I would allow it to be placed ( ie, built in a location) and after that it can't be moved.