Aren't both parties at fault here?
NWI at fault for offering such a strange deal on the face of it (targets are all very well, but how can you guarantee "x" no. of passengers will fly a particular route in a given timeframe?). And Flybe at fault for agreeing to it & thus risking a fine!
OK, you can FORECAST your passengers for the year ahead, but that's all it will EVER be - a forecast, NOT a guarantee.