in actuality, in my best educational guess, the reason why BMW did this is that, Diesel sales now are higher for BMW than those of Petrol engines in Europe that is. With the Euro becoming more stringent in CO2 emission, BMW diesel engine emits far less carbon as opposed to its gasoline counterpart. the fact that also diesel engines can produce so much torque specially when equipped with a turbo, it is sensible enough for them (IMHO) to do this.
We are used to the term ///M exclusively for Petrol engines but give it a few more years and the diesel engine can rev as high like those of the Petrol Engine. If you noticed the Le Mans 24 hours race, Audi is already using its TDi engines for the race and it can rev like the Petrol engines already.