Barney - you're looking for logic for one particular set of parameters.
The CAA had to draw the line somewhere, to define what could and could not be used for training. (Their lawyers seem to have had quite a hand in that, based on the letter we got). The line is where it is because it has to be somewhere, and that's where the bloke in charge decided to put it.
You could make an equally valid case for a range of places to draw the line. It just isn't our call...