These "photo contests" are ultimately nothing but a marketing instrument.
Certainly they cannot say that they would only grant the award to an image shot with a Nikon, but something tells me that by sheer coincidence the winning entries will be shot on Nikon.
They allowed the MF exception probably not to "devalue" the contest as an "amateur-only" one, and in general because even if a MF "wins" this is no big damage to the marketing effort.
But imagine a film image is going to win. How would that work as a marketing news? I imagine all the blogs and newspapers around the world: "last century technology beats modern cameras at Nikon contest".
Or imagine if an image - film or digital - wins and it is shot on let's say a Canon. The world would laugh.
Being Nikon, they cannot afford, from a sheer marketing point of view, to allow the contest to backfire.
You'll say: "it's the image, not the camera". Naahhhh The purpose of these contests is to imply, to hint, to suggest that "it's the camera". They cannot spend all the marketing effort and come out with a winner on film (or on a competitor's camera). That would be negating all their present marketing effort toward their digital line and toward their products.
The fact that it is all a marketing effort is well demonstrated by the 3rd and 4th cathegories, which are not even "photographic" in nature.
It's not very smart in any case. Smarter would have been allowing film, and not letting film win any category