FADEC failure 135  

#1 Too many people have lost their lives after shutting down the good engine on accident. 

#2 In a fadec failure the engine still runs and makes power, if that engine can be brought under manual control it's not a hazard to keep it running. In fact keeping it running helps you and gives you options in case there is an issue with the other engine which does require a emergency shut down like a engine fire or a complete engine failure.   

When the EP's are written they don't know if you'll be flying 100 miles off shore, or night IMC over the mountains to a high DA LZ, or on a day VFR flight with lots of forced landing areas.  Shutting down an engine removes options for the pilot.

Remember our #1 priority is to maintain NR. You're correct that keeping an engine with a fadec failure running results in a higher workload for the pilot, but that engine is one of only two sources of sustained NR which is what's keeping the helicopter flying.