I suspect Memory Map's issue is that the iOS APIs don't include a way of telling whether a location device is even connected, never mind controlling which device should be used. All they say is "location is X, with an accuracy of y". You then need to work out whether the accuracy is good enough (or the phone is actually using cell tower triangulation)...
Since their App was one of the first (if not thee first) on the market to use a GPS on iOS, it will have been written before the other libraries were available. For them to get more detailed GPS info, they will need to re-write that section of their code, but using other GPS libraries, which is probably not trivial. The libraries are specific to each GPS type, so may not work with future released external devices. From a developer's point of view, that will add an additional cost to manage and support the app.
I doubt its rocket science though - and to me it seems a must have feature!