Prof. Janet Smith is, arguably, this country's foremost moral theologian on issues having to do with sexuality. Here's her analysis of the "Congo nuns" situation. (Smith takes for granted the truth of the story, by the way).
There is an implication to the way Smith analyzes the story: "Rome" could have simply utilized the above analysis for the nuns who asked--thus "giving permission" under the specific outlined circumstances.
Or, as the legal beagles will say: it's facts and circumstances, folks.