Why does the ATR 72 that virgin flies starts the right engine and not allow the prop to spin?

I just have a question Why does the ATR 72 that virgin flies starts the right engine and not allow the prop to spin

The ATR doesn't have an APU as such to provide power to the aircraft on the ground when the engines shutdown, compared to other larger aircraft the have a seperate dedicated engine (APU) at the rear to generate power. Therefore the ATR uses its actual engine as a power sourse inplace of an APU and holds the prop from spinning. I believe they call it "hotel mode"

That's interesting VA773ER. I recalled a National Geographic Air Crash Investigation episode about the crash of Air Ontario Fokker 28 being partly due to the non-operational APU.

Didn't think modern aircraft could get away with not having an APU. Do the pilots turn off the engine of the ATR72 for refuelling? 

The crash your refering to wasn't caused by the defective APU itself.

The APU is used as a power source to start the main engines and to provide power to the aircraft while on the ground if there is no external power available at the location. Once the engines have started, power to the aircracft is now generated by the engines and the APU is shutdown as it is no longer needed.

In the case of the Air Ontario Fokker 28, the APU was non-operational AND there was no external power at the location, therefore they could not shutdown the engines or else they wouldn't have any power source to restart them. The problem was the aircraft needed to be de-iced but this cannot occur while the engines are running. So they didn't de-ice properly and the result was reduced lift/airflow, increased weight, this was the cause of the crash. :)

Also regarding refueling, yes the engines are turned off for refueling. At this point the aircraft is plugged into an external power source at the airport so running the engine in "hotel mode" isn't needed :)

Just out of curiosity, does this limit the destinations that the ATR72 can fly to in Australia or are all Australian airports equiped with this external power source? 

One of the problems for that Air Ontario flight was that the regional airport it flew into did not have an external power source.

I'm not sure on ports in Australia without external power but the ATR, being a smaller prop plane, is capable of starting its engines on battery power alone so if there are ports without external power, there shouldn't be a problem with restarting them. However since standard practice with many airlines is to start the ATR engines using external power, I think it would come down to individual company policies and wether or not they would allow it.